plpgsql's exec_simple_cast_value() mistakenly supposed that it could bypass

plpgsql's exec_simple_cast_value() mistakenly supposed that it could bypass
casting effort whenever the input value was NULL.  However this prevents
application of not-null domain constraints in the cases that use this
function, as illustrated in bug #4741.  Since this function isn't meant
for use in performance-critical paths anyway, this certainly seems like
another case of "premature optimization is the root of all evil".

Back-patch as far as 8.2; older versions made no effort to enforce
domain constraints here anyway.