[3.12] gh-116881: Remove erroneous or redundant grammar NULL (GH-116885) (#116951)

In Lexical Analysis f-strings section, NULL in the description
of 'literal character' means '\0'.  In the format_spec grammar
production, it is wrong with that meaning and redundant if
instead interpreted as <nothing>.  Remove it there.
(cherry picked from commit 4e45c6c54a9457b1ca5b4cf3aa2843b7218d4414)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>