Improve send-as behaviour

Receiving mails for wildcard alias addresses is really easy – but
sending mails from those any-aliases was not possible at all unless
every sender address was added as an explicit alias to the database.

By this change in the database query for allowed sender addresses, the
first finding `not NULL` (see [`SELECT COALESCE`](https://www.w3schools.com/sql/func_sqlserver_coalesce.asp) for how it works)
– either an exact alias `mailbox@domain.tld` or the wildcard alias `@domain.tld`
will be allowed to send mails as the given address ... without the need
of explicit definition within the database.