Rolling upgrade to set the welcome_user_email network-wide option via the...

Rolling upgrade to set the welcome_user_email network-wide option via the welcome_user_msg_filter() callback. Fixes ability to save the welcome user email when no entry exists in the database. see #20116

git-svn-id: https://core.svn.wordpress.org/trunk@20697 1a063a9b-81f0-0310-95a4-ce76da25c4cd