Fix #291 Null values in filters

[jburton-scottlogic]

Fixes #291

Added a check to ignore filters where the operand is null or undefined.
Added associated test to check for this functionality.

[texodus]

Thanks for the PR and test! Please send us a CLA form

I'm inclined to think "x > null" should be an error in the perspective module, and just never submitted in this state from the perspective-viewer module - I made a new issue to capture this #349