t1400: use test_must_be_empty

t1400: use test_must_be_empty

Use test_must_be_empty instead of reading the file and comparing its
contents to an empty string.  That's more efficient, as the function
only needs built-in meta-data only check in the usual case, and provides
nicer debug output otherwise.

Signed-off-by: René Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>