handle when PhEDEx return se null

Need to find out why PhEDEx returns se null.

https://cmsweb.cern.ch/phedex/datasvc/json/prod/blockreplicas?block=/ZJetsToNuNu_HT-400to600_Tune4C_13TeV-madgraph-tauola/Spring14dr-PU20bx25_POSTLS170_V5-v1/AODSIM%23136ecc80-0582-11e4-9e28-003048f02c8a