Fix #11025

Before in [`user_directory_handler._handle_deltas`, we just checked for `is_support_user(user_id)`](https://github.com/matrix-org/synapse/pull/10960/files#diff-e02a9a371e03b8615b53c6b6552f76fc7d3ef58931ca64d28b3512caf305449fL232) which works just fine.
Now with #10960, we [call `should_include_local_user_in_dir`](https://github.com/matrix-org/synapse/pull/10960/files#diff-e02a9a371e03b8615b53c6b6552f76fc7d3ef58931ca64d28b3512caf305449fR229) which does a [bunch of additional checks](https://github.com/matrix-org/synapse/blob/e79ee48313404abf8fbb7c88361e4ab1efa29a81/synapse/storage/databases/main/user_directory.py#L382-L398) which aren't all compatible with the main appservice sender (main bridge user/sender). More specifically, we can't check the `users` database table for whether the user is deactivated.

In the `should_include_local_user_in_dir` checks, we should return early if we encounter a main appservice sender before the incompatible checks.

…rowing-exception-when-interacting-with-appservice-sender

I don't think this actually improves coverage as such, but I'll sleep
better at night having this case checked!

That's the current behaviour (I didn't realise they typically don't
match their own user regex).