[3.12] gh-119614: Fix truncation of strings with embedded null characters in Tkinter (GH-120909) (GH-120939)

Now the null character is always represented as \xc0\x80 for
Tcl_NewStringObj().
(cherry picked from commit c38e2f64d012929168dfef7363c9e48bd1a6c731)

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