Fixed #20660 -- Do not try to delete an unset FieldFile

Thanks stanislas.guerra at gmail.com for the report and
Baptiste Mispelon for the review.

django/db/models/fields/files.py

@@ -96,6 +96,8 @@ class FieldFile(File):
     save.alters_data = True
 
     def delete(self, save=True):
+        if not self:
+            return
         # Only close the file if it's already open, which we know by the
         # presence of self._file
         if hasattr(self, '_file'):