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
1804284042e. Back-patch to 11.
Reported-by: Paul Schaap
Diagnosed-by: Andrew Gierth
Dicussion: https://postgr.es/m/15475-
11a7a783fed72a36%40postgresql.org