The WS dies with a 500:
[error] Caught exception in MusicBrainz::Server::Controller::WS::1::Artist->search "name is a required parameter at /home/musicbrainz/musicbrainz-server/script/../lib/MusicBrainz/Server/Data/Search.pm line 646."
It should send a 400 instead.
This is available for testing at http://test.musicbrainz.org
and is in for code review at http://codereview.musicbrainz.org/r/1246/