scripts: compliance: Avoid exception when commit message body is empty

Avoid the following exception in the Identity test when the commit body
is empty:

Traceback (most recent call last):
  File "zephyr/scripts/ci/check_compliance.py", line 2053, in main
    n_fails = _main(args)
              ^^^^^^^^^^^
  File "zephyr/scripts/ci/check_compliance.py", line 1988, in _main
    test.run()
  File "zephyr/scripts/ci/check_compliance.py", line 1459, in run
    auth_name, auth_email, body = git(
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: not enough values to unpack (expected 3, got 2)

This is triggered because ``%b`` represents the body of the commit
message, which can be empty if there's a single line in it, because the
first line is considered the title.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>