Setting an empty "owner" is allowed in the REST API

[EricWittmann]

Description

When updating the owner of an artifact, we are allowing an empty string. That should not be a valid value and rejected by the server.

Registry
Version: 2.4.3
Persistence type: all

Steps to Reproduce

1. Create a new artifact
2. Try changing the owner to "" (empty string)
3. It's allowed?!

This should not be allowed - it should fail with some sort of 4xx error, perhaps a 400. The error message should indicate that there is a missing parameter.

[EricWittmann]

The starting point for this is here:

https://github.com/Apicurio/apicurio-registry/blob/main/app/src/main/java/io/apicurio/registry/rest/v2/GroupsResourceImpl.java#L317-L323