MediaLibrary: Don't launch a rescan if the parser isn't started

This means that the media library is still initializing, and the parser
will be started a bit later. Since the first thing it will do is to
restore all unfinished tasks, the tasks will still execute.
Fix #404
9 jobs for !581 with fix_contraint_violation in 13 minutes and 18 seconds (queued for 32 seconds)
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