Fix Empty given to notandthen; make notandthen 2.5x faster
Fix Empty given to notandthen; make notandthen 2.5x faster

postfix `without` compiles to notandthen, so fix and speedup affects
it as well.

Empty as arg disappears in the `+@foo` slurpy, so we use
nqp:e5ebf22ee143948f59230393bbf8c08b4497264b@foo` slurpy to avoid Empty
dissappearance.

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