title: Django ImageField overwrites existing path when empty
body: <p>So, that's the problem:
   I currently have a model:</p>
  
  <pre>
  class UserData(models.Model):
      avatar = models.ImageField(verbose_name='Avatar',upload_to='images/profile_pics',blank=True,null=True)
      doc_type = models.CharField(verbose_name='Document type',max_length=1,default='0')
  
  
  </pre>
  
  <p>And a form:</p>
  
  <pre>
  class UserCreationForm(forms.ModelForm):
      avatar = forms.ImageField(label='Avatar',required=False, error_messages = {'invalid':"Images only"}, widget=forms.FileInput)
      class Meta:
          model = UserData
  </pre>
  
  <p>So, the problem occurs when the user tries to edit his data. When no image is provided, the current image path in the db overwrites with the empty string. Is there any way to solve that problem?</p>