filter:null gives unexpected results

[jwildfire]

Current behaviour: If a user specifies filters:null the default filters are rendered.
Expected behaviour: If a user specifies filters:null no filters are rendered.

Side notes:

• null is probably a more resonable default for filters
• filter:[] doesn't render any filters (this is fine)

cc: @bzkrouse

[jwildfire]

Possible duplicate of #46

[jwildfire]

On second thought, I actually think the current behavior here makes sense. I think that a user specifying null should mean: "use the defaults" - the same as if the user not specified anything at all. On the other hand, I think it's reasonable for an empty array to mean: "Ignore the default; I don't want any filters".

On a related note, I think there is a case to be made that we've got too many filters rendered by default, but we can make a different issue for that if folks think it is needed.

[jwildfire]

Closing as won't fix. #46 is a related, but different issue, so going to handle that separately.