[BUGFIX] DatabaseConnection must be initialized

The install tool forgets to initialize the database connection.

Change-Id: Ib7355e4f931c7152a6d13df99ed550c1b61eacf9
Fixes: #56763
Releases: 6.2
Reviewed-on: https://review.typo3.org/28254
Reviewed-by: Andreas Fernandez
Tested-by: Andreas Fernandez
Reviewed-by: Markus Klein
Tested-by: Markus Klein