preset: Complains about not being able to serialize NULL string properties
Submitted by Sebastian Dröge
It checks for gst_value_serialize() to return non-NULL to decide if a property was successfully serialized or not. a NULL string is serialized as NULL though, which is probably correct.
Should probably add some special cases here to not fill the logs with useless things.
Another thing that should be added as a special case is the "parent" property. We can't realistically serialize that one anyway.