Bug: 0 value arguments are removed?

[krisbitney]

This PR prevents erasure of falsy variable values by changing a validation check from:

if (!variables[node.name.value]) {
        throw Error(`Missing variable: ${node.name.value}`);
      }

to:

if (variables[node.name.value] === undefined) {
        throw Error(`Missing variable: ${node.name.value}`);
      }

The PR also adds a unit test to confirm the parse-query algorithm does not throw when variable value is falsy.

Closes #485.