fix(handleSubmit): accept nullish outputs with disabled fields - #13999

Proposed Changes

A resolver's transformed value can be null or undefined. When a field is registered with disabled: true, handleSubmit tries to remove that field from the nullish result. This throws before onValid runs and prevents submit state from completing.

Skip disabled-field cleanup for a top-level nullish result. Pass the transformed value through unchanged and preserve the existing cleanup behavior for object results.

Add three cases covering null/undefined outputs, callback return values and completed submit state, plus top-level/nested disabled-field removal without mutating the resolver's original object. The two nullish cases fail against the original implementation.

Validation with Node 22:

• 133 suites, 1,835 unit tests and 26 snapshots passed.
• All 90 existing Chromium E2E tests passed with one worker and no retries, using a local browser/server configuration. E2E test files and assertions are unchanged.
• Type checking, type tests, lint, Prettier, builds, export checks and both API Extractor checks passed. No added lint warnings; the API report is unchanged.

Type of change

• Bug fix

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes