t1400: test that stdin -z update treats empty <newvalue> as zeros

This is the (slightly inconsistent) status quo; make sure it doesn't
change by accident.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1400-update-ref.sh

@@ -730,6 +730,13 @@ test_expect_success 'stdin -z fails update with bad ref name' '
 	grep "fatal: invalid ref format: ~a" err
 '
 
+test_expect_success 'stdin -z treats empty new value as zeros' '
+	git update-ref $a $m &&
+	printf $F "update $a" "" "" >stdin &&
+	git update-ref -z --stdin <stdin &&
+	test_must_fail git rev-parse --verify -q $a
+'
+
 test_expect_success 'stdin -z fails update with no new value' '
 	printf $F "update $a" >stdin &&
 	test_must_fail git update-ref -z --stdin <stdin 2>err &&