[3.12] gh-120211: Fix tkinter.ttk with Tcl/Tk 9.0 (GH-120213) (GH-120216)

* Use new methods for tracing Tcl variable.
* Fix Combobox.current() for empty combobox.
(cherry picked from commit d68a22e7a68ae09f7db61d5a1a3bd9c0360cf3ee)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>