fix: prefab processor dirty up port (up port)

[NoelStephensUnity]

This is an up-port of #3102

NetworkPrefabProcessor now marks the default NetworkPrefabsList ScriptableObject asset as dirty if there are new imports OR deletions.

The Unity Editor is now aware of the changes and able to write them to disk. This prevents the changes from being lost when the editor is quit and allows them to be tracked in version control.

Previously the script would only mark the list as dirty if there had been both new imports and deletions.

Changelog

• Fixed: Issue where NetworkPrefabProcessor would not mark the prefab list as dirty and prevent saving the DefaultNetworkPrefabs asset when only imports or only deletes were detected.

Testing and Documentation

• No tests have been added.
• No documentation changes or additions were necessary