[BUGFIX] stdWrap.hash: Return nothing for non-existing algorithm

Return an empty string to prevent disclosing information unhashed.

Change-Id: Id25b85de039797aa7e39225fb0e2b1c75207a505
Resolves: #28120
Reviewed-on: http://review.typo3.org/3324
Reviewed-by: Jo Hasenau
Tested-by: Jo Hasenau
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers