fix: Fixes warning logged on double-initialize of prefabs lists.

[ShadauxCat]

Also, makes adding prefabs before initializing the list no longer an error case - those prefabs are stored in a separate list so that during initialization, they can be re-added to the prefabs list after pulling in data from the NetworkPrefabsList ScriptableObjects.

Changelog

• Fixed: Fixed warning "Runtime Network Prefabs was not empty at initialization time." being erroneously logged when no runtime network prefabs had been added
• Changed: Adding network prefabs before NetworkManager initialization is now supported.

Testing and Documentation

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

[NoelStephensUnity]

LGTM!