Fix undeletion that runs into nullable ForeignKeys.
Fix undeletion that runs into nullable ForeignKeys.

mccarrmb/moztrap,mozilla/moztrap,bobsilverberg/moztrap,mozilla/moztrap,mccarrmb/moztrap,shinglyu/moztrap,bobsilverberg/moztrap,bobsilverberg/moztrap,bobsilverberg/moztrap,mccarrmb/moztrap,mccarrmb/moztrap,shinglyu/moztrap,mccarrmb/moztrap,mozilla/moztrap,shinglyu/moztrap,shinglyu/moztrap,mozilla/moztrap,shinglyu/moztrap,mozilla/moztrap