fix that JsonObjectBuilder.remove removes to much in certain cases

[thjaeckle]

This looks like a really small bug, but is has a huge impact:

• Currently, for ThingMerged events the payload value can get "lost" before persisting a merge because of this bug

Here an inline policy is removed from the to-be-persisted JSON:

ditto/things/service/src/main/java/org/eclipse/ditto/things/service/persistence/serializer/ThingMongoEventAdapter.java

Lines 51 to 53 in d008959

	if (event instanceof ThingMerged) {
	return super.performToJournalMigration(event, jsonObject)
	.remove(POLICY_IN_THING_MERGED_VALUE_PAYLOAD); // remove the policy entries from thing merged payload

The JsonPointer to be removed is /value/_policy - if this _policy however does not exist and if the value does not even contain a JsonObject (which can be the case when using "merge", e.g. only updating one specific path to e.g. a JsonNumber), then the current implementation of ImmutableJsonObjectBuilder.remove will remove the value from the event.
That leads to the event missing its value in persistence and that leads to much pain and suffering when replaying events for that persistence actor, throwing exceptions on deserialization if the value is missing.

[thjaeckle]

@alstanchev @kalinkostashki a review would be nice - this is a really fundamental issue when using "MergeThing" commands :/

[alstanchev]

LGTM 👍