Fix NULL handling in multi-batch Parallel Hash Left Join.

NULL keys in left joins were skipped when building batch files.
Repair, by making the keep_nulls argument to ExecHashGetHashValue()
depend on whether this is a left outer join, as we do in other
paths.

Bug #15475.  Thinko in 18042840.  Back-patch to 11.

Reported-by: Paul Schaap
Diagnosed-by: Andrew Gierth
Dicussion: https://postgr.es/m/15475-11a7a783fed72a36%40postgresql.org

src/backend/executor/nodeHashjoin.c

@@ -1395,7 +1395,7 @@ ExecParallelHashJoinPartitionOuter(HashJoinState *hjstate)
 		if (ExecHashGetHashValue(hashtable, econtext,
 								 hjstate->hj_OuterHashKeys,
 								 true,	/* outer tuple */
-								 false, /* outer join, currently unsupported */
+								 HJ_FILL_OUTER(hjstate),
 								 &hashvalue))
 		{
 			int			batchno;