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

[EugeneFlesselle]

If we reach the second case of Zip[T1 <: Tuple, T2 <: Tuple], then we know (T1, T2) is disjoint from (NonEmptyTuple, NonEmptyTuple), from which we can conclude at least one of the two is an EmptyTuple.

Addressing #19175

[EugeneFlesselle]

Well actually it looks like the doc was just incorrect:

scala3/library/src/scala/Tuple.scala

Lines 177 to 186 in 7a0230d

	/** 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`.
	*/
	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
	}

-- [E007] Type Mismatch Error: tests/playground/example.scala:10:59 ------------
10 |  val z: Tuple.Zip[Int *: Int *: Tuple, String *: Tuple] = (1, "a") *: (??? : Tuple)
   |                                                           ^^^^^^^^^^^^^^^
   |Found:    (Int, String) *: Tuple
   |Required: (Int, String) *: Tuple.Zip[Int *: Tuple, Tuple]
   |
   |Note: a match type could not be fully reduced:
   |
   |  trying to reduce  Tuple.Zip[Int *: Tuple, Tuple]
   |  failed since selector (Int *: Tuple, Tuple)
   |  does not match  case (h1 *: t1, h2 *: t2) => (h1, h2) *: Tuple.Zip[t1, t2]
   |  and cannot be shown to be disjoint from it either.
   |  Therefore, reduction cannot advance to the remaining cases
   |
   |    case (EmptyTuple, _) => EmptyTuple
   |    case Any => Tuple

even though the doc would suggest the match type reduces to (Int, String) *: Tuple.

[EugeneFlesselle]

@sjrd do you concur with logic in the commit message of f8e584d, as I wouldn't mind confirmation.