Do not autovivify Parameter's $!signature_constraint
Do not autovivify Parameter's $!signature_constraint

Parameter's internals assume this is nqp::null if absent. Doing so makes
them more readable and less apt to break in the future:
nqp::ifnull($!attr) vs. nqp::eqaddr($!attr, <type>) vs. the slower
nqp::defined($!attr).

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