gitweb: Make config_to_multi return [] instead of [undef]

gitweb: Make config_to_multi return [] instead of [undef]

This is important for the list of clone urls, where if there are
no per-repository clone URL configured, the default base URLs
are never used for URL construction without this patch.

Add tests for different ways of setting project URLs, just in case.
Note that those tests in current form wouldn't detect breakage fixed
by this patch, as it only checks for errors and not for expected
output.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>