[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: 6.2

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