[ticket/11372] Migrator should only check if effectively installed if not

installed at all

PHPBB3-11372
Merge remote-tracking branch 'EXreaction/ticket/11372' into develop

# By Nathaniel Guse
# Via Nathaniel Guse
* EXreaction/ticket/11372:
  [ticket/11372] Migrator should only check if effectively installed if not