title: Saving file field as None but reflects empty string in the database
body: <p>I'm trying to submit a form with a file field. There are instances when the file is not yet available so the user will have to submit the form without the file first.</p>
  
  <p>My field now looks like this</p>
  
  <pre><code>file_attachment_pdf = models.FileField(blank= True,null = True, unique=True)
  </code></pre>
  
  <p>I'm able to save 1 record with blank fields. When adding the 2nd record with blank file upload, it doesn't work anymore since there is an existing record with the file value. </p>
  
  <p>Upon checking, it's an empty string. </p>
  
  <p>So I tried to override the clean function in models.py</p>
  
  <pre><code>def clean(self):
  
          if not self.file_attachment_doc:
              print('doc is blank, will tag as Null')
              self.file_attachment_doc = None
  </code></pre>
  
  <p>But for some reason, when I check in the DB it still stores as an empty string. (I'm using DB Browser for SQLite, it says the field of data is Text with 0 chars)</p>
  
  <p>When I try to submit, here is the error that is returned to me</p>
  
  <p><strong>File with this File attachment pdf already exists.</strong></p>
  
  <p>If this question was already asked and answered before, please do let me know.</p>