fix: disable whitespace-only submissions

[521Peter]

Summary

• treat whitespace-only composer values as empty
• keep submit disabled during initial rendering, prop updates, and status changes
• add regression coverage for initial and updated whitespace-only messages

Testing

• yarn --cwd specs test specs/ui/js/Composer/Composer-typing.spec.ts
• yarn lint

Fixes #164

[vercel]

@521Peter is attempting to deploy a commit to the NLKit Team on Vercel.

A member of the Team first needs to authorize it.