Ignore blank lines in email diversion exceptions

An accidental blank line in divertallemailsexcept causes no emails to be diverted.

To reproduce:

1. Enter an email address in divertallemailsto and an extra blank line in divertallemailsexcept
2. Send a test email {siteurl}/admin/testoutgoingmailconf.php
3. Receive the test email without a diversion applied

The email in step 2 should be diverted because blank lines are easy to include in the multi-line entry for divertallemailsexcept and processing a blank line defeats the whole point of the setting. Therefore blank lines should be ignored.

To fix, this line needs changing:

https://github.com/moodle/moodle/blob/7ce003b666a66b465ce9335f430a6e4d3535a7f1/lib/moodlelib.php#L5876

To

        if ($pattern != "" and preg_match("/$pattern/", $email)) {