not changing feedback labels will remove them

When not updating feedback labels, an empty array is sent which leads to the labels being removed. With comment labels the current situation is that labels will be removed even when not sending the label field at all. The behaviour of the feedback labels is the correct but the frontend should omit the labels field when not editing labels. For the comment behaviour we should change the corresponding code in the feedback serializer's update method to match the behaviour of the feedback label endpoint, which is equivalent to the JSON Merge Format specification.

• Let frontend omit labels when nothing has changed
• Change the feedback serializer's update code to not replace the field with an empty array