DDF-94 null check for metadata field of metacards.
DDF-94 Added unit tests for how predicates handle null contextual data.  Added null checks into Contextual and Entry Predicates.
DDF-94 removed other potential NPE identified in code review.
DDF-94 additional unit tests for null operation.
Merge pull request #25 from codice/DDF-94

DDF 94: merge null check for metadata field of metacards