[ticket/11901] Pass empty string as relative_url for non-local magic URL...

Only the regular expression for MAGIC_URL_LOCAL has three capturing groups. As
a result $matches[3] is only set in this case. Prior to PHPBB3-11606 an empty
string was passed to make_clickable_callback() as a replacement in case of less
than three capturing groups. This patch reintroduces the empty string.

Regression from 49c12ef4be229bf2223139298766ef441b075fbc.

PHPBB3-11901

http://tracker.phpbb.com/browse/PHPBB3-11901