Don't worry about empty tuple types in TupleToIntersection. (#4502)

Although a zero-argument call to `mergeDeep` was always pretty pointless,
it seemed worth handling that case for the sake of completeness.

Well, apparently empty tuple types are forbidden by section 3.8.5 of the
TypeScript spec, so (some versions of) the compiler complain about the
`[]` in the `T extends []` case of `TupleToIntersection`:
https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#385-tuple-type-literals

Since we never really needed this case in the first place, the easy
solution is to remove it.

Note that the final cases in `TupleToIntersection` continue to cover the
zero-argument case, for whatever it may be worth:

  T extends (infer U)[] ? U : any

Should fix #4501.

packages/apollo-utilities/src/util/mergeDeep.ts

@@ -15,7 +15,6 @@ const { hasOwnProperty } = Object.prototype;
 // true & false, and the inferred type ends up as unknown in many cases),
 // in addition to being nearly impossible to explain/understand.
 export type TupleToIntersection<T extends any[]> =
-  T extends [] ? {} :
   T extends [infer A] ? A :
   T extends [infer A, infer B] ? A & B :
   T extends [infer A, infer B, infer C] ? A & B & C :