Fixed void material for Python API
Fixed void material for Python API

lilulu/openmc,wbinventor/openmc,kellyrowland/openmc,amandalund/openmc,amandalund/openmc,shikhar413/openmc,shikhar413/openmc,johnnyliu27/openmc,paulromano/openmc,mjlong/openmc,amandalund/openmc,shikhar413/openmc,samuelshaner/openmc,walshjon/openmc,wbinventor/openmc,walshjon/openmc,mit-crpg/openmc,mit-crpg/openmc,wbinventor/openmc,mit-crpg/openmc,smharper/openmc,paulromano/openmc,liangjg/openmc,johnnyliu27/openmc,walshjon/openmc,liangjg/openmc,liangjg/openmc,bhermanmit/openmc,wbinventor/openmc,kellyrowland/openmc,mit-crpg/openmc,bhermanmit/openmc,amandalund/openmc,paulromano/openmc,mjlong/openmc,walshjon/openmc,samuelshaner/openmc,johnnyliu27/openmc,johnnyliu27/openmc,shikhar413/openmc,samuelshaner/openmc,smharper/openmc,lilulu/openmc,smharper/openmc,lilulu/openmc,smharper/openmc,liangjg/openmc,samuelshaner/openmc,paulromano/openmc