Fix sorting so that all null and undefined values stay at bottom
Fix sorting so that all null and undefined values stay at bottom

yeatmanlab/AFQ-viz,yeatmanlab/AFQ-Browser,yeatmanlab/AFQ-Browser,arokem/AFQ-Browser,arokem/AFQ-Browser,richford/AFQ-Browser,yeatmanlab/AFQ-Browser,richford/AFQ-viz,richford/AFQ-Browser,yeatmanlab/AFQ-viz,richford/AFQ-viz,richford/AFQ-Browser,arokem/AFQ-Browser