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.

Reported-by: Andreas Bühmann <dev@uuml.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

builtin/for-each-repo.c

@@ -51,6 +51,9 @@ int cmd_for_each_repo(int argc, const char **argv, const char *prefix)
 	values = repo_config_get_value_multi(the_repository,
 					     config_key);
 
+	if (!values)
+		return result;
+
 	for (i = 0; !result && i < values->nr; i++)
 		result = run_command_on_repo(values->items[i].string, &args);
 

t/t0068-for-each-repo.sh

@@ -27,4 +27,8 @@ test_expect_success 'run based on configured value' '
 	grep again message
 '
 
+test_expect_success 'do nothing on empty config' '
+	git for-each-repo --config=bogus.config -- these args would fail
+'
+
 test_done