Refs #34517 -- Restored skipping ImageFileField.update_dimension_fields without width/height fields.

This avoids reading the image size when the dimensions fields
(image_width, image_height) do not exist, as that operation may be
expensive.

Partially reverts ea53e7c09f, that dropped
the check for the dimension fields in update_dimension_fields(), because
the post_init signal was no longer registered without dimension fields.

However, another code path to that function exists: when the
ImageFileField is save()d, the name from the storage is setattr()ed on
the field, and ImageFileDescriptor calls update_dimension_fields()
because the image size might have changed. Keep bailing out early when
dimensions are unused.

Besides, computing the image dimensions causes to close() the file,
resulting in a backward-incompatible change. The test protects against
that change.

tests/model_fields/test_imagefield.py

@@ -336,6 +336,13 @@ class ImageFieldNoDimensionsTests(ImageFieldTwoDimensionsTests):
             [sender_id for (_, sender_id), *_ in signals.post_init.receivers],
         )
 
+    def test_save_does_not_close_file(self):
+        p = self.PersonModel(name="Joe")
+        p.mugshot.save("mug", self.file1)
+        with p.mugshot as f:
+            # Underlying file object wasn’t closed.
+            self.assertEqual(f.tell(), 0)
+
 
 @skipIf(Image is None, "Pillow is required to test ImageField")
 class ImageFieldOneDimensionTests(ImageFieldTwoDimensionsTests):