Fix UserMessageToBfWrite and UserMessageToBfRead

These two stocks have their behavior reversed, they currently return `null` on non-protobuf messages, but it should be the other way around.

This currently causes an issue in [funcommands blind/drugs](https://mxr.alliedmods.net/sourcemod/source/plugins/funcommands/blind.sp#67) where a TNE gets thrown during usermessage creation, which then later brings down the entire server process since the usermessage is never completed.