BUG #12994. vtkXMLMultiBlockDataWriter not writing names for leaf nodes.

vtkXMLMultiBlockDataWriter was not saving out names for leaf nodes the the
mulitblock dataset. Names were only being saved for non-leaf nodes. Fixed that.

Change-Id: If68a32d64ad5eab2c5f107a236cba6dfecfe125d
Merge topic '12994-xmlmultiblockwriter-improvements' into master

3a192370 BUG #12994. vtkXMLMultiBlockDataWriter not writing names for leaf nodes.