[BUGFIX] DBAL: Incomplete database configuration

When no database connection is established, handler_init() is used. But
at this point, the credentials are unkown and the method returns void.

As all parameters of method sql_pconnect() are deprecated, move the
_DEFAULT initialization into method handler_init() to get credentials
into the DBAL configuration.

Resolves: #56257
Releases: 6.2
Change-Id: Ifd908fab1382434d376a9358d0ae9c3af083616e
Reviewed-on: https://review.typo3.org/28045
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Andreas Fernandez
Tested-by: Andreas Fernandez
Reviewed-by: Markus Klein
Tested-by: Markus Klein