Use null values when elements get explicitly set to no value

[StefanApfel-Bentley]

Whenever a schema element has been not set in the source schema, the differences should use null as value. This way, things that hasn't changed remain undefined, while null is telling there was a change, but it unset the value.

There might be a problem with the schema difference bodies, which currently use the SchemaJSON props as content and they don't have null value.