fix: truncate when reverting writes during NetworkBehaviour synchronize

[NoelStephensUnity]

This fixes an issue where a NetworkBehaviour would add an additional 6 bytes to its serialization data if nothing was written during the synchronize call.

MTT-7719

Changelog

• Fixed: Issue where NetworkBehaviour.Synchronize was not truncating the write buffer if nothing was serialized during NetworkBehaviour.OnSynchronize causing an additional 6 bytes to be written per NetworkBehaviour component instance.

Testing and Documentation

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