Allow "null" to be passed in filters

[richmolj]

And convert to Ruby's nil

[richmolj]

Did some initial work here: https://github.com/richmolj/graphiti/tree/filter_nil

Though the work is pretty straightforward, there is a purpose to disallowing nils. Passing a nil can sometimes cause slow/expensive database queries, and it should probably be disabled by default. This means we probably need 2 options

• filter :foo, allow_nil: true
• self.filters_accept_nil_by_default = true (defaults to false`)

@wadetandy sound about right?

[wadetandy]

Makes sense and I agree that we should not change the defaults. If we're doing some nice things on behalf of nil, it might be a good idea to do the same for javascript's undefined, though I can also see this being a more common thing that is actual a desired filter. Maybe have a configurable list of "strings we coerce to nil" with sensible defaults?

[zeisler]

I would recommend following the JSON spec that undefined is an invalid conversion to Ruby.

> JSON.parse("undefined")
  #=> JSON::ParserError (416: unexpected token at 'undefined')
> JSON.parse("null")
  #=> nil

[richmolj]

Yeah, I agree with @zeisler - also makes sense because in Spraypaint, I would drop undefined and not send as part of the request, but null should turn into the string "null".

Maybe we'll move to a configurable whitelist at some point, but that seems good for now.

[wadetandy]

In general I would Be in favor of either allowing nil and undefined OR allowing neither, but I’m not going to get too worked up about any outcome here.