fix blank newline in XML output
fix blank newline in XML output

While working with vtkPhyloXMLTreeWriter, I noticed that it would
occasionally output blank lines.  This commit fixes this behavior,
resulting in a more concise output file.

Change-Id: I3bd848198ea299246c1ef4d1f4a6ccea515729bf

demarle/VTK,candy7393/VTK,mspark93/VTK,SimVascular/VTK,sumedhasingla/VTK,sankhesh/VTK,ashray/VTK-EVM,jmerkow/VTK,candy7393/VTK,gram526/VTK,ashray/VTK-EVM,candy7393/VTK,candy7393/VTK,demarle/VTK,berendkleinhaneveld/VTK,hendradarwin/VTK,berendkleinhaneveld/VTK,hendradarwin/VTK,sankhesh/VTK,sumedhasingla/VTK,berendkleinhaneveld/VTK,mspark93/VTK,msmolens/VTK,candy7393/VTK,hendradarwin/VTK,johnkit/vtk-dev,keithroe/vtkoptix,sankhesh/VTK,johnkit/vtk-dev,jmerkow/VTK,SimVascular/VTK,gram526/VTK,berendkleinhaneveld/VTK,SimVascular/VTK,jmerkow/VTK,johnkit/vtk-dev,sumedhasingla/VTK,berendkleinhaneveld/VTK,mspark93/VTK,hendradarwin/VTK,msmolens/VTK,sumedhasingla/VTK,ashray/VTK-EVM,demarle/VTK,msmolens/VTK,ashray/VTK-EVM,hendradarwin/VTK,jmerkow/VTK,jmerkow/VTK,demarle/VTK,sumedhasingla/VTK,johnkit/vtk-dev,demarle/VTK,demarle/VTK,msmolens/VTK,sankhesh/VTK,keithroe/vtkoptix,sumedhasingla/VTK,sankhesh/VTK,sankhesh/VTK,ashray/VTK-EVM,keithroe/vtkoptix,msmolens/VTK,gram526/VTK,johnkit/vtk-dev,keithroe/vtkoptix,jmerkow/VTK,sankhesh/VTK,candy7393/VTK,SimVascular/VTK,jmerkow/VTK,ashray/VTK-EVM,demarle/VTK,keithroe/vtkoptix,hendradarwin/VTK,johnkit/vtk-dev,gram526/VTK,berendkleinhaneveld/VTK,candy7393/VTK,SimVascular/VTK,msmolens/VTK,demarle/VTK,SimVascular/VTK,msmolens/VTK,sumedhasingla/VTK,berendkleinhaneveld/VTK,msmolens/VTK,candy7393/VTK,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,mspark93/VTK,gram526/VTK,keithroe/vtkoptix,gram526/VTK,jmerkow/VTK,ashray/VTK-EVM,mspark93/VTK,hendradarwin/VTK,gram526/VTK,mspark93/VTK,keithroe/vtkoptix,johnkit/vtk-dev,keithroe/vtkoptix,gram526/VTK,mspark93/VTK,ashray/VTK-EVM,sankhesh/VTK,mspark93/VTK