[TASK] Clean up DataMapper::convertClassNameToTableName

[TASK] Clean up DataMapper::convertClassNameToTableName

This removes pointless code which ends up doing strtolower(NULL).

Also make the class name argument mandatory since it does not make
sense to pass nothing or NULL and the core does not do this in any
case.

The latter change makes this mildly breaking if code relies on the
old behavior.

Resolves: #67248
Releases: master
Change-Id: I3f3339ea005831fc3d78974d8883c20ad04ec4ec
Reviewed-on: http://review.typo3.org/39900
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>