fix issue with "shareadraft" plugin

See comments here:
https://github.com/HBehrens/podlove-publisher/commit/01acddde990da1fd2721c6d18d70250cd037de67#commitcomment-4163856

Basically, there is an exception handling in the code which only applies
to the migration assistant - but affects other plugins. So the easy fix
is to only do it when the migration assistant module is actually active.

commit 01acddde990da1fd2721c6d18d70250cd037de67