[3.12] gh-127208: Reject null character in _imp.create_dynamic() (#127400) (#127419)

gh-127208: Reject null character in _imp.create_dynamic() (#127400)

_imp.create_dynamic() now rejects embedded null characters in the
path and in the module name.

Backport also the _PyUnicode_AsUTF8NoNUL() function.

(cherry picked from commit b14fdadc6c620875a20b7ccc3c9b069e85d8557a)