In `BP_Messages_Thread::is_valid()` add default value for `$thread_id` and perform an empty check inside the method. This allows `messages_is_valid_thread()` to return a proper result if an empty ID is passed into it.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8620 cdf35c40-ae34-48e0-9cc9-0c9da1808c22