Add null test to partition constraint for default range partitions.

Add null test to partition constraint for default range partitions.

Non-default range partitions have a constraint which include null
tests, and both default and non-default list partitions also have a
constraint which includes null tests, but for some reason this was
missed for default range partitions.  This could cause the partition
constraint to evaluate to false for rows that were (correctly) routed
to that partition by insert tuple routing, which could in turn
cause constraint exclusion to prune the default partition in cases
where it should not.

Amit Langote, reviewed by Kyotaro Horiguchi

Discussion: http://postgr.es/m/ba7aaeb1-4399-220e-70b4-62eade1522d0@lab.ntt.co.jp