[BUGFIX] BackendUtility::calcAge returns negative value for 0

Currently the function calcAge returns the value -0 min for zero seconds.
This should be changed as zero is unsigned. Besides some UnitTests are
added to verify the functionality of that function.

Change-Id: Ie0c7572e63bf8d3a6f074f2ccf0db63ac6f5c801
Fixes: #46984
Releated: 6.1, 6.0
Reviewed-on: https://review.typo3.org/19670
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn