for-each-repo: do nothing on empty config

for-each-repo: do nothing on empty config

'git for-each-repo --config=X' should return success without calling any
subcommands when the config key 'X' has no value. The current
implementation instead segfaults.

A user could run into this issue if they used 'git maintenance start' to
initialize their cron schedule using 'git for-each-repo
--config=maintenance.repo ...' but then using 'git maintenance
unregister' to remove the config option. (Note: 'git maintenance stop'
would remove the config _and_ remove the cron schedule.)

Add a simple test to ensure this works. Use 'git help --no-such-option'
as the potential subcommand to ensure that we will hit a failure if the
subcommand is ever run.

Reported-by: Andreas Bühmann <dev@uuml.de>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>