Fixed issue 15924: Can not send invitations to imported participants where emailstatus IS NULL (#1395)

Dev: although the WHERE condition "emailstatus <> 'OptOut'" intuitively should match everything that is not equal to "OptOut" it actually does not match NULL values in MySQL since it is not NULL-safe.

Dev: This is related to 15772 and 15823 since emailstatus should not be NULL in the first place.