Skip pidFilter if "null"

Addresses DataOne MN Checker AssertionError: should still get a LogEntry when applying 'pidFilter' parameter [for host GET http://datadryad.org/mn/v1/log?toDate=2013-02-19T15:22:04.273%2B00:00&pidFilter=null ]
Merge pull request #72 from datadryad/trello-9-dataonemn

Skip pidFilter if "null"