(bug 45069) wfParseUrl() no longer produces a PHP notice if passed a "mailto:" URL...

(bug 45069) wfParseUrl() no longer produces a PHP notice if passed a "mailto:" URL without address

Calling wfParseUrl( 'mailto:' ) or wfParseUrl( 'mailto:?subject=...' ) was resulting in the following PHP errors:
PHP Notice:  Undefined index: path in includes/GlobalFunctions.php on line 813
PHP Notice:  Undefined index: path in includes/GlobalFunctions.php on line 814

Now the existence of that key is checked and if missing an empty string is set so for consistency of the returned value.

The fix is based on the snippet provided on
https://www.mediawiki.org/wiki/Thread:Project:Support_desk/GLobafunctions.php_wfParseUrl_choking_on_url

Change-Id: Ife07d6e2a364a7cafda387cf7ed9cd71c2b68ef8