title: Django Admin set extra to 0 when editing
body: <p>Setting <code>extra = 1 </code> in my model always shows a 1 empty field. It is OK while inserting a new item but I don't want to show an additional empty field while editing. How can I do this?</p>
  <p><strong>models.py</strong>:</p>
  <pre><code>class Foo(models.Model):
      bar = models.ForeignKey(Bar, models.CASCADE, related_name='bars')
      title = models.CharField(_('Title'), max_length=255)
      body = models.TextField(_('Body'))
      __str__(self):
          return '%s' % (self.title)
  </code></pre>
  <p><strong>admin.py</strong>:</p>
  <pre><code>class FooInline(admin.StackedInline):
      model = Foo
      extra = 1 #Also shows 1 extra empty field while editing.
                #I don't want to show if there is already a non-empty field
  
  class FooAdmin(admin.ModelAdmin):
      prepopulated_fields = {'slug': ('title',)}
      inlines = [FooInline]
  </code></pre>