keymap_ui: Replace `zed::NoAction` with `null` (#34562)

Closes #ISSUE

This change applies both to the UI (we render `<null>` as muted text
instead of `zed::NoAction`) as well as how we update the keymap file
(the duplicated binding is bound to `null` instead of `"zed::NoAction"`)

Release Notes:

- N/A *or* Added/Fixed/Improved ...
573836a654Ben Kunkle committed on 7/16/2025, 5:55:58 PM· committed by GitHubparent048dc47
2 files changedLine totals unavailable