[BUGFIX] Install tool uses empty database settings

Some systems allow to connect with empty username / password
configuration to the database. Although this is not wrong the install
tool should always ask for database credentials for new installations.
This patch prevents the install tool from setting an empty username and
password as default values without manually submitting the form.

Resolves: #51540
Resolves: #51956
Releases: 6.2
Change-Id: Ic245635bf649f4ad477d793eda658096ed659aa0
Reviewed-on: https://review.typo3.org/23866
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Markus Klein
Tested-by: Markus Klein