Fix incorrectStringCompare cppcheck warning
Fix incorrectStringCompare cppcheck warning

Fixed warning:
String literal "NULL" doesn't match length argument for substr().

Change-Id: I91641f2f2edb678c3d557d4d00307e96953fb5a4

sumedhasingla/VTK,sumedhasingla/VTK,demarle/VTK,sumedhasingla/VTK,sumedhasingla/VTK,candy7393/VTK,demarle/VTK,SimVascular/VTK,sankhesh/VTK,gram526/VTK,jmerkow/VTK,keithroe/vtkoptix,jmerkow/VTK,SimVascular/VTK,sankhesh/VTK,demarle/VTK,mspark93/VTK,candy7393/VTK,mspark93/VTK,SimVascular/VTK,msmolens/VTK,candy7393/VTK,gram526/VTK,sankhesh/VTK,demarle/VTK,sankhesh/VTK,msmolens/VTK,msmolens/VTK,SimVascular/VTK,mspark93/VTK,keithroe/vtkoptix,jmerkow/VTK,sankhesh/VTK,msmolens/VTK,candy7393/VTK,gram526/VTK,msmolens/VTK,gram526/VTK,candy7393/VTK,keithroe/vtkoptix,mspark93/VTK,SimVascular/VTK,sumedhasingla/VTK,jmerkow/VTK,jmerkow/VTK,msmolens/VTK,sumedhasingla/VTK,keithroe/vtkoptix,mspark93/VTK,candy7393/VTK,demarle/VTK,mspark93/VTK,gram526/VTK,sumedhasingla/VTK,gram526/VTK,gram526/VTK,jmerkow/VTK,keithroe/vtkoptix,msmolens/VTK,msmolens/VTK,sankhesh/VTK,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,candy7393/VTK,mspark93/VTK,jmerkow/VTK,sankhesh/VTK,sumedhasingla/VTK,jmerkow/VTK,keithroe/vtkoptix,demarle/VTK,gram526/VTK,SimVascular/VTK,keithroe/vtkoptix,demarle/VTK,candy7393/VTK,mspark93/VTK,sankhesh/VTK,demarle/VTK