title: Null value violates not-null constraint, but model has blank=True null=True
body: <p>I have a form</p>
  
  <pre class="lang-py prettyprint-override"><code>class EmployeeHistoryForm(forms.ModelForm):
      department = forms.ModelChoiceField(
          label="Подразделение",
          queryset=Department.objects.all()
          )
      post = forms.ModelChoiceField(
          label="Должность",
          queryset=Post.objects.all()
          )
      rank = forms.IntegerField(
          label="Разряд",
          validators=[
              MaxValueValidator(MAX_RANK),
              MinValueValidator(1)
          ])
      start_date = forms.DateField(
          label="Дата начала работы",
          initial=datetime.now().date()
          )
  
      class Meta:
          model = EmployeeHistory
          exclude = ['employee', 'end_date']
  </code></pre>
  
  <p>to model</p>
  
  <pre class="lang-py prettyprint-override"><code>class EmployeeHistory(models.Model):
      employee = models.ForeignKey(Employee, on_delete=models.CASCADE)
      department = models.ForeignKey(Department, on_delete=models.CASCADE)
      post = models.ForeignKey(Post, on_delete=models.CASCADE)
      rank = models.IntegerField(
          validators=[
              MaxValueValidator(MAX_RANK),
              MinValueValidator(1)
          ]
      )
      start_date = models.DateField()
      end_date = models.DateField(blank=True, null=True)
  </code></pre>
  
  <p><code>employee</code> field is filled from another form and I want to keep <code>end_date</code> None-typed for a while.</p>
  
  <p>This is view:</p>
  
  <pre class="lang-py prettyprint-override"><code>def add_employee_action(request):
      if request.method == "POST":
          ...
          history_form = EmployeeHistoryForm(request.POST)
          if personal_form.is_valid() and history_form.is_valid():
              ...
              employee.save()
  
              history=EmployeeHistory(
                      employee=employee,
                      department=Department.objects.filter(
                          pk=request.POST['department']
                      )[0],
                      post=Post.objects.filter(
                          pk=request.POST['post']
                      )[0],
                      rank=request.POST['rank'],
                      start_date=datetime.now().date(),
                      end_date=None
                  )
              history.save()
      else:
          ...
          history_form = EmployeeHistoryForm()
      return render(
          request,
          'add_employee.html',
          context={
              ...
              'history_form': history_form,
              }
      )
  </code></pre>
  
  <p>But when I submit the form have a <code>django.db.utils.IntegrityError: null value in column "end_date" violates not-null constraint
  DETAIL:  Failing row contains (7, 12, 2019-10-26, null, 1, 10, 1).</code> I'm using PostgreSQL.</p>
  
  <p><strong>Note:</strong> I've added <code>blank=True, null=True</code> after first migration, then migrate again. May be this will importaint.</p>