getMissingRequiredVariables report missing falsy yet valid values

[theobat]

Hi @mattkrick,

I found a problem while trying to query something containing a floor in a very basic form. Floor is expected to be a non null Int but very often equals 0. I found out that the falsy values are not handled in
getMissingRequiredVariables. This PR solves the problem by specifically looking for null or undefined.

Edit: coverage decreases because the queryHelpers.js file was not in the tests percentage before and I only test one of the few functions of the file

[coveralls]

Coverage decreased (-4.01%) to 72.388% when pulling 1d97208 on theobat:master into da2e138 on mattkrick:master.

[theobat]

Also if you don't have time to review this in the near future please let me know so that I use my fork as a dependency instead. Thanks.
@mattkrick