Build an iterator to efficiently handle null/not null checks in the IteratorBuildingVisitor

[ivakegg]

A specialty iterator is needed which handles null/not null checks against non-event fields to allow their usage in queries. The iterator would need to return at most one key per document, then either seek/next to the following document. This could likely be done with a custom FieldIndexAggregator that is substituted in when a null/not null value is specified.