Handle case when textDims are NULL
Handle case when textDims are NULL

Add a NULL pointer check to handle the case when textDims are not provided in
vtkFreeTypeTools::RenderStringInternal().

This fixes a regression introduced in commit 5dcad97.

SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix