cmd/cloner: add regression test for slice nil/empty semantics

We had a misstep with the semantics when applying an optimization that
showed up in the roll into corp. This test ensures that case and related
cases must be retained.

Updates #9410
Updates #9601
Signed-off-by: James Tucker <james@tailscale.com>

cmd/cloner/clonerex/clonerex.go

@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:generate go run tailscale.com/cmd/cloner  -clonefunc=true -type SliceContianer
+
+package clonerex
+
+type SliceContianer struct {
+	Slice []*int
+}