Short-circuit row estimation in NOT IN containing NULL consts

Short-circuit row estimation in NOT IN containing NULL consts

ScalarArrayOpExpr used for either NOT IN or <>/= ALL, when the array
contains a NULL constant, will never evaluate to true.  Here we add an
explicit short-circuit in scalararraysel() to account for this and return
0.0 rows when we see that a NULL exists.  When the array is a constant,
we can very quickly see if there are any NULL values and return early
before going to much effort in scalararraysel().  For non-const arrays,
we short-circuit after finding the first NULL and forego selectivity
estimations of any remaining elements.

In the future, it might be better to do something for this case in
constant folding.  We would need to be careful to only do this for
strict operators on expressions located in places that don't care about
distinguishing false from NULL returns. i.e. EXPRKIND_QUAL expressions.
Doing that requires a bit more thought and effort, so here we just fix
some needlessly slow selectivity estimations for ScalarArrayOpExpr
containing many array elements and at least one NULL.

Author: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>
Reviewed-by: David Geier <geidav.pg@gmail.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/eaa2598c-5356-4e1e-9ec3-5fd6eb1cd704@tantorlabs.com