(PE-1452) Allow querying for containing-class=null

Prior to this commit, you could not query for containing-class = null.
This commit provides support for querying against null, and it also
allows for other fields (property, message, file, line) to now be
queried for null.
(PE-1452) Explicitly check for nil
Merge pull request #650 from shakedown/dev/burgundy/pe1452-query-null-support

(PE-1452) Allow querying for containing-class=null