Treat provably empty slices of variadic tuples as empty - #22211

Slicing a variadic tuple where start and stop count from the same end and start >= stop gives an empty tuple at runtime, whatever the tuple's length. mypy reports an error instead:

from typing import TypeVarTuple

Ts = TypeVarTuple("Ts")

def f(x: tuple[int, *Ts, str], y: tuple[int, *tuple[str, ...]]) -> None:
    reveal_type(x[2:1])  # error: Ambiguous slice of a variadic tuple; Revealed type is "Any"
    reveal_type(y[5:2])  # same
    reveal_type(x[-1:-2])  # same

TupleType.slice only covers a few prefix/suffix layouts for variadic tuples. When begin is past the unpack, it falls through to the return None branch, and the caller reports the ambiguous-slice error. Fixed tuples don't have this problem because they just use Python's own slicing.

If begin and end are both non-negative or both negative, then begin >= end (step positive or omitted) or begin <= end (step negative) means the slice is empty for every possible length, because Python clamps both bounds the same way. The fix handles that case first and returns tuple[()]. All other cases behave as before; the test checks that y[1:5] and y[-2:-5:-1] still report the ambiguous-slice error.

Test: testVariadicTupleEmptySlice in check-typevar-tuple.test. It fails on master.