[BUGFIX] Installer fails when no database is selected

Display an error message, if the selected database name is empty.

Resolves: #54569
Releases: 6.2
Change-Id: Idbf768d7edc0e5d5e1d0faf87d62e1060c70b1e9
Reviewed-on: https://review.typo3.org/26544
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Oliver Klee
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind