fix: handle null or empty named message registration [MTT-7771]

[NoelStephensUnity]

This catches the case where trying to register or "unregister" a null or empty string will log an error and not throw an exception.
This also resolves the issue on the NetworkTransform side where, under certain conditions, it was possible for NetworkTransform to attempt to "unregister" its named message before it had been assigned.

MTT-7771

fix: #2772

Changelog

• Changed: CustomMessageManager will no longer attempt to register or "unregister" a null or empty string and will log an error if this condition occurs.
• Fixed: Issue where NetworkTransform could potentially attempt to "unregister" a named message prior to it being registered.

Testing and Documentation

• Includes unit test.
• No documentation changes or additions were necessary.

[zachstronaut]

Is it correct that this fix is not part of the 1.8.0 release?

[NoelStephensUnity]

That is correct. v1.8.0 was delayed a bit so there could be some catch up on the documentation.
The next update will contain this fix.