Combine cases of Tuple.Zip disjoint from (h1 *: t1, h2 *: t2)

If we reach the second case of `Zip[T1 <: Tuple, T2 <: Tuple]`,
then we know `T1` and `T2` are both disjoint from `NonEmptyTuple`.
From which I believe we can conclude they are both subtypes of `EmptyTuple`.

I went with `(EmptyTuple, EmptyTuple)` for the 2nd pattern,
although I believe it should be equivalent to a wildcard pattern,
for the same reason as described above.

Author: EugeneFlesselle

Diff for: library/src/scala/Tuple.scala

@@ -175,14 +175,13 @@ object Tuple {
   }
 
   /** Given two tuples, `A1 *: ... *: An * At` and `B1 *: ... *: Bn *: Bt`
-   *  where at least one of `At` or `Bt` is `EmptyTuple` or `Tuple`,
    *  returns the tuple type `(A1, B1) *: ... *: (An, Bn) *: Ct`
-   *  where `Ct` is `EmptyTuple` if `At` or `Bt` is `EmptyTuple`, otherwise `Ct` is `Tuple`.
+   *  where `Ct` is `EmptyTuple` if `At` and `Bt` are `EmptyTuple`,
+   *  otherwise `Ct` is stuck as `Zip[At, Bt]`.
    */
   type Zip[T1 <: Tuple, T2 <: Tuple] <: Tuple = (T1, T2) match {
     case (h1 *: t1, h2 *: t2) => (h1, h2) *: Zip[t1, t2]
-    case (EmptyTuple, _) => EmptyTuple
-    case (_, EmptyTuple) => EmptyTuple
+    case (EmptyTuple, EmptyTuple) => EmptyTuple
   }
 
   /** Converts a tuple `(F[T1], ..., F[Tn])` to `(T1,  ... Tn)` */