graphql: pass through nil instead of erroring on undefined variable
graphql: pass through nil instead of erroring on undefined variable

From the spec: http://facebook.github.io/graphql/October2016/#sec-The-__InputValue-Type
`null` is the correct default value for a variable that is not provided and
does not have its own default value. This will allow clients to pass in
arguments as `undefined` (i.e. omitted from the json variables block)
without subtlely breaking queries.

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