[3.12] gh-106359: Fix corner case bugs in Argument Clinic converter parser (GH-106361) (#106364)

gh-106359: Fix corner case bugs in Argument Clinic converter parser (GH-106361)

DSLParser.parse_converter() could return unusable kwdicts in some rare cases

(cherry picked from commit 0da4c883cf4185efe27b711c3e0a1e6e94397610)

Co-authored-by: Erlend E. Aasland <erlend@python.org>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>