Creating a Thing with PATCH (merge patch) semantic will retain null values which is invalid

[thjaeckle]

In Ditto it is possible to also create Things with Merge updates.
This either updates an existing thing or creates it when it does not yet exist.

The implemented Merge Patch RFC 7396 defines that null values have a special meaning: they are used to delete fields from a JSON.

When Ditto however creates the Thing if it was not yet existing when doing a PATCH, this semantic is not applied which is not adhering to the RFC.
Ditto must therefore "delete" also JSON fields with null values when creating a not-yet existing Thing on a PATCH request.