[#5078] Fix: Don't use NULL as third parameter in mb_substr

git-svn-id: https://svn.typo3.org/TYPO3v4/Extensions/oelib/trunk@85680 735d13b6-9817-0410-8766-e36946ffe9aa
[#5079] Change: Add dependencies for the test extensions

git-svn-id: https://svn.typo3.org/TYPO3v4/Extensions/oelib/trunk@85681 735d13b6-9817-0410-8766-e36946ffe9aa