Make NULL tuple values always advance skip arrays.

_bt_check_compare neglected to handle a case that can arise when the
scan's keys are temporarily treated as nonrequired, as an optimization:
whenever a NULL tuple value was encountered that had a skip array whose
current element wasn't already NULL, _bt_check_compare failed to advance
the array to the NULL element.  This allowed _bt_check_compare to fail
to return matching tuples containing a NULL value (though only with an
array column that came before a skip array column with NULLs, and only
during _bt_readpage calls that set pstate.forcenonrequired=true on a
page where the higher-order column also had to advance).

To fix, teach _bt_check_compare to handle this case just like any other
case where a skip array key is unsatisfied and must be advanced directly
(due to the key being considered a nonrequired key).

Oversight in commit 8a510275, which optimized nbtree search scan key
comparisons with skip arrays.

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://postgr.es/m/CAHgHdKtLFWZcjr87hMH0hYDHgcifu4Tj7iHz-xh8qsJREt5cqA@mail.gmail.com

src/backend/access/nbtree/nbtutils.c

@@ -2920,6 +2920,17 @@ _bt_check_compare(IndexScanDesc scan, ScanDirection dir,
 
 		if (isNull)
 		{
+			/*
+			 * Scalar scan key isn't satisfied by NULL tuple value.
+			 *
+			 * If we're treating scan keys as nonrequired, and key is for a
+			 * skip array, then we must attempt to advance the array to NULL
+			 * (if we're successful then the tuple might match the qual).
+			 */
+			if (unlikely(forcenonrequired && key->sk_flags & SK_BT_SKIP))
+				return _bt_advance_array_keys(scan, NULL, tuple, tupnatts,
+											  tupdesc, *ikey, false);
+
 			if (key->sk_flags & SK_BT_NULLS_FIRST)
 			{
 				/*
@@ -2958,7 +2969,7 @@ _bt_check_compare(IndexScanDesc scan, ScanDirection dir,
 			}
 
 			/*
-			 * In any case, this indextuple doesn't match the qual.
+			 * This indextuple doesn't match the qual.
 			 */
 			return false;
 		}