Enforce route variable function if there are non-null arguments

[AlecAivazis]

A query like the following should not require a variable function

query AboutQuery($first: Int, $after: String, $last: Int, $before: String { 
   ...
}

[fehnomenal]

Probably also when the query has only default arguments (nullable or not).

[AlecAivazis]

I'm removing the bug tag since this no longer errors. The preprocessor now looks for the variable functions existence before generating a reference to it. While it did cover the original scenario, we did lose a "fast fail" since the user has to look at the network response to see why the error failed.

We now need a check that a variable function exists if the query has non-nullable inputs