title: Django save field as NULL
body: <p>I have form fields, I would like to keep them empty sometimes.
  The problem is that save() method in the views is expecting the filed and throwing error.</p>
  
  <p><strong>my forms:</strong></p>
  
  <pre><code>from django import forms
  from cProfile import label
  
  
  class DocumentForm(forms.Form):
      docfile = forms.FileField(
          label='Select a file'
      )
  
      time_from = forms.DateTimeField(
          label = 'select range of time, from'
      )
  
      time_to = forms.DateTimeField(
          label = 'to:', required = False
      )
  </code></pre>
  
  <p><strong>my models:</strong></p>
  
  <pre><code>from django.db import models
  import uuid
  from django.utils import timezone
  from celery.worker.strategy import default
  
  
  class Document(models.Model):
      docfile = models.FileField(upload_to='documents/%Y/%m/%d')
      id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
      time_from = models.DateTimeField(default=None, null=True, blank=True)
      time_to = models.DateTimeField(default=None, null=True, blank=True)
  </code></pre>
  
  <p><strong>my views:</strong></p>
  
  <pre><code>def list(request):
      # Handle file upload
      if request.method == 'POST':
          form = DocumentForm(request.POST, request.FILES)
          if form.is_valid():
              newdoc = Document(docfile=request.FILES['docfile'], time_from=request.POST['time_from'], time_to=request.POST['time_to'])
              newdoc.save()
  </code></pre>
  
  <p>I am trying to keep <strong>time_to</strong> field empty when submitting, but it is giving me this error:</p>
  
  <blockquote>
    <p>Exception Value: [u"'' value has an invalid format. It must be in
    YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format."]</p>
  </blockquote>
  
  <p>I think the reason is that <strong>newdoc.save()</strong> is expecting the DateTimeField of <strong>time_to</strong> format as <strong>u'2017-05-05 12:02:02'</strong> and getting an empty string or something like: <strong>u''</strong></p>