t6429: fix use of non-existent function

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>