feat: Prevent empty reply objects to have properties (planning#1293)

Events with empty objects - example delete-reply - should have no other properties.

I validated it with an Online JSON Schema Validator:

IS:

SHOULD:

requested review from @Jonas_Groeger

assigned to @Christoph_Metzger

@Jonas_Groeger should we add "additionalProperties": false to each *-submission event with empty object as well (example delete-submission)? Or should we do it later with planning#1306 (closed) when zustelldienst guarantees to create only valid events.

Discussed with @Jonas_Groeger this topic. For now we do not implement the option "additionalProperties": false. In future changes (e.g. SET payload schema v2.0.0) we can think of making it excplicit with "additionalProperties": false.

closed