Scrip-setup+ts: ensure proper handling of `null` as default prop value.

Editor assessment

This one has several small spec and style problems despite a valid `fix(types)` header. 'scrip-setup' is a typo for 'script-setup', and embedding 'scrip-setup+ts:' as a pseudo-scope inside the subject is redundant when a real scope could carry it, for example `fix(compiler-sfc)`. The subject ends with a period, which the conventional style drops, and it drifts past a comfortable length. The footer `fix #4868` uses a lowercase token; the spec permits any token and GitHub will still link the issue, so that part is fine. Most importantly, the body is just the reference — a line describing how `null` default prop values were mishandled would make this far more useful to future readers.