[BUGFIX] Check for query failures in admin methods

In the DatabaseConnection class the two methods admin_get_fields
and admin_get_keys throw a fatal error when a query failure occurs.

This fix introduces two checks that prevent the methods from fataling
and return an empty result on failure.

Fixes: #53014
Releases: 6.0, 6.1, 6.2

Change-Id: I40f7e52fe14b7d75766f0484e42834b0e1b1a5dc
Reviewed-on: https://review.typo3.org/24940
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-on: https://review.typo3.org/25484
Reviewed-by: Francois Suter
Tested-by: Francois Suter