title: Error in import FloatField, using django-import-export
body: <p>I am using django-import-export for import csv file. I have a <code>FloatField</code> in my model :</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>purchase_price = models.FloatField(null=True, blank=True)
  </code></pre>
  
  <p>When I import csv file with blank value, it throws an error :</p>
  
  <p><strong>ValueError at /admin/csv_imp/book/process_import/</strong></p>
  
  <p>could not convert string to float: </p>
  
  <pre><code>Request Method:     POST
  Request URL:    http://localhost:8000/admin/csv_imp/book/process_import/
  Django Version:     1.7.1
  Exception Type:     ValueError
  Exception Value:    
  
  could not convert string to float: 
  
  Exception Location:     /home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py in get_prep_value, line 1550
  Python Executable:  /home/bgdev/Desktop/virtualenvs/django17/bin/python
  Python Version:     2.7.6
  Python Path:    
  
  ['/home/bgdev/Desktop/virtualenvs/django17/test_pro',
   '/home/bgdev/Desktop/virtualenvs/django17/src/admin-bootstrap',
   '/home/bgdev/Desktop/virtualenvs/django17/lib/python2.7',
   '/home/bgdev/Desktop/virtualenvs/django17/lib/python2.7/plat-i386-linux-gnu',
   '/home/bgdev/Desktop/virtualenvs/django17/lib/python2.7/lib-tk',
   '/home/bgdev/Desktop/virtualenvs/django17/lib/python2.7/lib-old',
   '/home/bgdev/Desktop/virtualenvs/django17/lib/python2.7/lib-dynload',
   '/usr/lib/python2.7',
   '/usr/lib/python2.7/plat-i386-linux-gnu',
   '/usr/lib/python2.7/lib-tk',
   '/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages',
   '/home/bgdev/Desktop/virtualenvs/django17/lib/python2.7/site-packages']
  
  Server time:    Tue, 9 Dec 2014 05:33:36 +0000
  
  
  
  Environment:
  
  
      Request Method: POST
      Request URL: http://localhost:8000/admin/csv_imp/book/process_import/
  
      Django Version: 1.7.1
      Python Version: 2.7.6
      Installed Applications:
      ('bootstrap_admin',
       'import_export',
       'django.contrib.admin',
       'django.contrib.auth',
       'django.contrib.contenttypes',
       'django.contrib.sessions',
       'django.contrib.messages',
       'django.contrib.staticfiles',
       'csv_imp')
      Installed Middleware:
      ('django.contrib.sessions.middleware.SessionMiddleware',
       'django.middleware.common.CommonMiddleware',
       'django.middleware.csrf.CsrfViewMiddleware',
       'django.contrib.auth.middleware.AuthenticationMiddleware',
       'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
       'django.contrib.messages.middleware.MessageMiddleware',
       'django.middleware.clickjacking.XFrameOptionsMiddleware')
  
  
      Traceback:
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
        111.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
        105.                     response = view_func(request, *args, **kwargs)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
        52.         response = view_func(request, *args, **kwargs)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/contrib/admin/sites.py" in inner
        204.             return view(request, *args, **kwargs)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/import_export/admin.py" in process_import
        130.                                  raise_errors=True)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/import_export/resources.py" in import_data
        359.                     six.reraise(*sys.exc_info())
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/import_export/resources.py" in import_data
        345.                         self.save_instance(instance, real_dry_run)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/import_export/resources.py" in save_instance
        163.             instance.save()
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/base.py" in save
        591.                        force_update=force_update, update_fields=update_fields)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/base.py" in save_base
        619.             updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/base.py" in _save_table
        681.                                       forced_update)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/base.py" in _do_update
        725.         return filtered._update(values) &gt; 0
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/query.py" in _update
        600.         return query.get_compiler(self.db).execute_sql(CURSOR)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py" in execute_sql
        1004.         cursor = super(SQLUpdateCompiler, self).execute_sql(result_type)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py" in execute_sql
        775.             sql, params = self.as_sql()
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py" in as_sql
        969.                 val = field.get_db_prep_save(val, connection=self.connection)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py" in get_db_prep_save
        627.                                       prepared=False)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py" in get_db_prep_value
        619.             value = self.get_prep_value(value)
      File "/home/bgdev/Desktop/virtualenvs/django17/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py" in get_prep_value
        1550.         return float(value)
  
      Exception Type: ValueError at /admin/csv_imp/book/process_import/
      Exception Value: could not convert string to float: 
  </code></pre>