Allow returning of Nil from Foo:D constraints
Allow returning of Nil from Foo:D constraints

In all cases. This was allowed already in the main fast path, but the
fallback path needed to behave consistently.

Fixes #2201.

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