Apollo codegen should create an empty variables interface for queries with no variables

**Intended outcome:**

When we have queries that don't use variables, we should have an empty interface. We can then use that interface, even if it's empty, in the variables generic for all apollo fetch methods.

The issue does not name any files, tests, or entry points. Start by locating the Apollo codegen path that creates TypeScript variables types for queries, then verify generated output for a query without variables and confirm that an empty interface is emitted and usable by Apollo fetch methods.

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