rev-parse: fix segfault with missing --path-format argument

rev-parse: fix segfault with missing --path-format argument

Calling "git rev-parse --path-format" without an argument segfaults
instead of giving an error message. Commit fac60b8925 (rev-parse: add
option for absolute or relative path formatting, 2020-12-13) added the
argument parsing code but forgot to handle NULL.

Returning an error makes sense here because there is no default value we
could use. Add a test case to verify.

Signed-off-by: Wolfgang Müller <wolf@oriole.systems>
Signed-off-by: Junio C Hamano <gitster@pobox.com>