fix: Objects without observers are DestroyWithOwner

[EmandM]

• Ensure objects with no observers (or only current owner as observer) are treated as if DontDestroyWithOwner = true on client disconnect
• Explicitly check if childObject has new client as observer on client connect

MTTB-1041
Changelog for this is in #3323.

Changelog

• Fixed: Ensure objects with no observers are treated as if DontDestroyWithOwner = true on client disconnect

Testing and Documentation

• No tests have been added.

[NoelStephensUnity]

😸