Support empty string for wgEmergencyContact/wgPasswordSender

In previous versions, the installer often outputted the following
in the generated LocalSettings.php:

$wgEmergencyContact = '';
$wgPasswordSender = '';

While this case did not result in providing default values in recent
MediaWiki versions, the mail handling didn't cause an error.

As of MediaWiki 1.25, the error handling is more strict and these
values being empty causes a fatal error and breaks all outgoing mail.

Bug: T104142
Change-Id: Ibf1f857b2f250dac9b725aff8f442e08b8ecd5c9