t6429: fix use of non-existent function

This test had a line reading ! test_file_is_empty actual which was meant to be ! test_must_be_empty actual The test worked despite the error, because even though test_file_is_empty is a non-existent function, the '!' negated the return value and made it pass. It'd be better to avoid the negation, so something like test_file_not_empty actual would be better, but perhaps it makes even more sense to specify the number of lines of expected output to make the test a bit tighter. Reported-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Elijah Newren <newren@palantir.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>