Improve tests for whether we can skip queueing RI enforcement triggers.

During an update of a PK row, we can skip firing the RI trigger if any old
key value is NULL, because then the row could not have had any matching
rows in the FK table.  Conversely, during an update of an FK row, the
outcome is determined if any new key value is NULL.  In either case it
becomes unnecessary to compare individual key values.

This patch was inspired by discussion of Vik Reykja's patch to use IS NOT
DISTINCT semantics for the key comparisons.  In the event there is no need
for that and so this patch looks nothing like his, but he should still get
credit for having re-opened consideration of the trigger skip logic.