Web installer: always autoselect some DB type

If the PHP mysql extension were not available, none of the DB type
radio buttons would be selected, and if the form were submitted
without selecting any of them, a PHP fatal error would occur
because of a lack of input validation.

So have the installer autoselect a DB type other than mysql if
necessary, and add a simple check to prevent a fatal error if all
radio buttons nevertheless end up deselected for some unknown reason.

Bug: 47489
Change-Id: Ic456899028c054a761d172df8ec32f6a26dc5b97