Fix Baggy:U ~~ Baggy:D emitting gut-referencing warnings
Fix Baggy:U ~~ Baggy:D emitting gut-referencing warnings

The smartmatch uses (<+) op under the hood to compare the two Baggies that
compares them by using > op over key's values. The key lookup of a Baggy:U
returns an Any, which causes a warning to emit about undef in num comparison.

Add extra candidates to (<+) to handle :U cases, equating a Baggy:U to being
an empty Baggy:D (reasoning: Bag.keys has same empty list as Bag.new.keys).

Note: for mirror symmetry, the same type of handling has been added to (+>) op,
but not other setty/baggy ops that most definitely need the same treatment to
avoid issuing gut-referencing warnings.

lucasbuchala/rakudo,samcv/rakudo,tbrowder/rakudo,tbrowder/rakudo,tony-o/rakudo,jonathanstowe/rakudo,lucasbuchala/rakudo,softmoth/rakudo,Gnouc/rakudo,samcv/rakudo,softmoth/rakudo,Gnouc/rakudo,awwaiid/rakudo,salortiz/rakudo,MasterDuke17/rakudo,usev6/rakudo,tbrowder/rakudo,salortiz/rakudo,nbrown/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,jonathanstowe/rakudo,samcv/rakudo,softmoth/rakudo,tony-o/rakudo,labster/rakudo,usev6/rakudo,tony-o/rakudo,salortiz/rakudo,rakudo/rakudo,jonathanstowe/rakudo,lucasbuchala/rakudo,salortiz/rakudo,labster/rakudo,rakudo/rakudo,lucasbuchala/rakudo,nbrown/rakudo,softmoth/rakudo,raydiak/rakudo,tony-o/rakudo,samcv/rakudo,labster/rakudo,tbrowder/rakudo,nbrown/rakudo,awwaiid/rakudo,Gnouc/rakudo,labster/rakudo,labster/rakudo,nbrown/rakudo,Gnouc/rakudo,rakudo/rakudo,lucasbuchala/rakudo,rakudo/rakudo,awwaiid/rakudo,Gnouc/rakudo,sjn/rakudo,raydiak/rakudo,usev6/rakudo,sjn/rakudo,raydiak/rakudo,nbrown/rakudo,Gnouc/rakudo,sjn/rakudo,jonathanstowe/rakudo,jonathanstowe/rakudo,awwaiid/rakudo,tbrowder/rakudo,tony-o/rakudo,salortiz/rakudo,rakudo/rakudo,salortiz/rakudo,tony-o/rakudo,sjn/rakudo,awwaiid/rakudo,raydiak/rakudo,usev6/rakudo,softmoth/rakudo,tbrowder/rakudo,usev6/rakudo,samcv/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,labster/rakudo,nbrown/rakudo,rakudo/rakudo,sjn/rakudo