Fix one more function return type confusion

In the same spirit of upstream commit c74d586d, but in an area of
Greenplum specific LDAP handling code.

For context, the return type of parse_hba_line changed in
postgres/postgres@e5e2fc842c4.

src/backend/libpq/hba.c

@@ -1362,7 +1362,7 @@ parse_hba_line(List *line, int line_num)
 					 errmsg("cannot use 'ldaptls' or 'ldapport' with 'ldapserver' start with 'ldaps://'"),
 					 errcontext("line %d of configuration file \"%s\"",
 								line_num, HbaFileName)));
-			return false;
+			return NULL;
 
 		}
 	}