tests: use 'test_must_be_empty' instead of 'test ! -s'

tests: use 'test_must_be_empty' instead of 'test ! -s'

Using 'test_must_be_empty' is preferable to 'test ! -s', because it
gives a helpful error message if the given file is unexpectedly no
empty, while the latter remains completely silent.  Furthermore, it
also catches cases when the given file unexpectedly does not exist at
all.

This patch was created by:

  sed -i -e 's/test ! -s/test_must_be_empty/' t[0-9]*.sh

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>