Don't write to postgres's stdin in shmem startup failure test

The single-user test expects postgres to fail during startup, before
processing any queries. Sending the preceding test's queries can race
with that exit and make IPC::Run abort on a broken pipe. To fix, pass
empty input instead.

Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://www.postgresql.org/message-id/e87c8fef-e254-441e-b889-bbcbd5afea7f@gmail.com
Backpatch-through: 19

src/test/modules/test_shmem/t/001_late_shmem_alloc.pl

@@ -215,7 +215,7 @@ SELECT test_shmem_register('test_shmem after startup', -1, 3);
 			'-c' => 'test_shmem.area_size=-1',
 			'postgres'
 		],
-		'<' => \$queries,
+		'<' => \'',
 		'2>' => \$startup_stderr);
 	ok(!$result,
 		"unknown-size request during single-user startup is rejected");