Change value of Pair.AT-KEY on non-existent key
Change value of Pair.AT-KEY on non-existent key

Currently the code returns a Mu, which isn't accepted by most operators
and causes show-stopping and confusing errors.

Fix by returning a much more friendly Nil instead.

Discussion: https://irclog.perlgeek.de/perl6-dev/2017-01-23#i_13971026

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