[BUGFIX] DataMapFactory::resolveTableName must remove leading backslashes

Currently the method just splits the given class name
by backslashes to detect the proper vendor name. If using
a leading backslash the algorithm of this function causes
a wrong result. Therefore all leading backslashes must be
removed first.

Releases: 6.2, 6.1, 6.0
Fixes: #52708
Change-Id: Icdc46facf66260bf528994964fb713d3dd24b3c6
Reviewed-on: https://review.typo3.org/24596
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Stefan Froemken
Tested-by: Stefan Froemken
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring