If user enters a regex without a first group (as the default regex), it will fail at https://github.com/metabrainz/picard/blob/60442412e7dfa7ff45f905bedd2653ad69d979b9/picard/coverart/providers/local.py#L101 with exception `IndexError: no such group`

https://community.metabrainz.org/t/please-help-us-test-picard-2-7-beta-1/554211/12