MDL-44377 dml: enforce non-empty prefix for sqlsrv

Only MySQL is allowed to run Moodle instances without
(with an empty) database prefix. The rationale is that
"user" is a reserved word in every RDBMS but MySQL.

Before this patch, installation was breaking badly, when
the "user" CREATE TABLE statement was issued. With this
patch the error is shown earlier (on connection) preventing
any advance/use of the database.

This check is present in all the drivers, just sqlsrv was missing it.