Don't add concreteness guard if return value = Nil
Don't add concreteness guard if return value = Nil

The guard set doesn't end up matching the values being passed, so the
plugin is repeatedly called when it shouldn't be.

Gnouc/rakudo,nbrown/rakudo,MasterDuke17/rakudo,softmoth/rakudo,tony-o/rakudo,rakudo/rakudo,labster/rakudo,Gnouc/rakudo,rakudo/rakudo,nbrown/rakudo,Gnouc/rakudo,nbrown/rakudo,tony-o/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,salortiz/rakudo,nbrown/rakudo,labster/rakudo,MasterDuke17/rakudo,tony-o/rakudo,salortiz/rakudo,MasterDuke17/rakudo,rakudo/rakudo,softmoth/rakudo,softmoth/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,Gnouc/rakudo,tbrowder/rakudo,softmoth/rakudo,rakudo/rakudo,salortiz/rakudo,nbrown/rakudo,tony-o/rakudo,Gnouc/rakudo,tony-o/rakudo,rakudo/rakudo,tbrowder/rakudo,labster/rakudo,nbrown/rakudo,salortiz/rakudo,labster/rakudo,labster/rakudo,labster/rakudo,tbrowder/rakudo,softmoth/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,rakudo/rakudo,salortiz/rakudo,salortiz/rakudo,tony-o/rakudo,Gnouc/rakudo