Fix null pointer use in constructing string


git-svn-id: https://root.cern.ch/svn/root/trunk@48783 27541ba8-7e3a-0410-8455-c3a389f83636
Missed a spot in r48782


git-svn-id: https://root.cern.ch/svn/root/trunk@48784 27541ba8-7e3a-0410-8455-c3a389f83636