Introducing NULL and EMPTY operators

[gmourier]

We introduced the EXISTS operator with v0.29.

EXISTS checks for the existence of a field. Fields with empty or null values still count as existing. The expression release_date NOT EXISTS returns all documents without a release_date

Adding a granular way to filter documents by field having null value or being considered empty e.g "", [] or {} could be useful especially for documents management when it will be possible to delete documents by a filter expression.

Related to #539, #89 and #325

Edit: closed in favor of #539