Handle empty query variables

[TimothyDJewell]

When testing the build of defining-mutations, queries no longer worked. The console revealed the error: Uncaught SyntaxError: Unexpected end of JSON input. After reviewing, this was because the default value for the query variables (the empty string) was being passed directly to JSON.parse. While looking into this, I noticed that a simple fix was introduced for this in 07a5f14 on build-schema (which I've copied here) and that master also handles this case, but in a very different way.

There is a simple workaround of just typing {} into the query variables text box, but this is a simple solution to just make it work out of the box.