Do not omit empty fields from JSON

[brightbyte]

The new serialization code, Serializers\EntitySerializer, omits empty fields from the data structure. E.g. if there are no descriptions set, addDescriptionsToSerialization() will do nothing instead of adding $serialization['descriptions'] = array();

This is a breaking change versus the old serialization code, compare getSerialized() in Wikibase\Lib\Serializers\EntitySerializer.

This leads to some parts of the JSON representation being optional, which forces consumers to always check for preses. This is likely to break existing consumers (bots, gadgets, etc).

[brightbyte]

https://bugzilla.wikimedia.org/show_bug.cgi?id=66791