POFile:+translate OOPSes when loading a suggestion with no POFile

POFile:+translate OOPSes like OOPS-179ec78410b8af059254cb47ab76a328 when it attempts to preloadDetails on a suggestion that doesn't have a corresponding POFile.

In the only known case so far, POTMsgSet 3203927 ("Delete") exists in POTemplate 14011 (https://launchpad.net/listen/0.4/+pots/listen) and has a TranslationMessage for Language 540 (en_GB), but there's no POFile for Language 540 in POTemplate 14011.

     Module lp.translations.model.translationmessage, line 565, in preloadDetails
    ['potemplateID'])
    Module lp.services.database.bulk, line 170, in load_related
    keys.update(map(partial(getattr, owning_object), foreign_keys))

AttributeError: 'NoneType' object has no attribute 'potemplateID'

This is a regression introduced by my preloadDetails work.