Fix handling of NULLs when merging BRIN summaries

When merging BRIN summaries, union_tuples() did not correctly update the
target hasnulls/allnulls flags. When merging all-NULL summary into a
summary without any NULL values, the result had both flags set to false
(instead of having hasnulls=true).

This happened because the code only considered the hasnulls flags,
ignoring the possibility the source summary has allnulls=true.

Discovered while investigating issues with handling empty BRIN ranges
and handling of NULL values, but it's a separate problem (has nothing to
do with empty ranges).

Fixed by considering both flags on the source summary, and updating the
hasnulls flag on the target summary.

Backpatch to 11. The bug exists since 9.5 (where BRIN indexes were
introduced), but those releases are EOL already.

Discussion: https://postgr.es/m/9d993d0d-e431-2196-9ccc-0554d0e60154%40enterprisedb.com

src/backend/access/brin/brin.c

@@ -1623,8 +1623,11 @@ union_tuples(BrinDesc *bdesc, BrinMemTuple *a, BrinTuple *b)
 
 		if (opcinfo->oi_regular_nulls)
 		{
+			/* Does the "b" summary represent any NULL values? */
+			bool		b_has_nulls = (col_b->bv_hasnulls || col_b->bv_allnulls);
+
 			/* Adjust "hasnulls". */
-			if (!col_a->bv_hasnulls && col_b->bv_hasnulls)
+			if (!col_a->bv_allnulls && b_has_nulls)
 				col_a->bv_hasnulls = true;
 
 			/* If there are no values in B, there's nothing left to do. */
@@ -1636,12 +1639,17 @@ union_tuples(BrinDesc *bdesc, BrinMemTuple *a, BrinTuple *b)
 			 * values from B into A, and we're done.  We cannot run the
 			 * operators in this case, because values in A might contain
 			 * garbage.  Note we already established that B contains values.
+			 *
+			 * Also adjust "hasnulls" in order not to forget the summary
+			 * represents NULL values. This is not redundant with the earlier
+			 * update, because that only happens when allnulls=false.
 			 */
 			if (col_a->bv_allnulls)
 			{
 				int			i;
 
 				col_a->bv_allnulls = false;
+				col_a->bv_hasnulls = true;
 
 				for (i = 0; i < opcinfo->oi_nstored; i++)
 					col_a->bv_values[i] =