@react-types/shared/collections: fix return types of KeyboardDelegate

[dlech]

As noted in the doc comments, the return values for each of the methods in KeyboardDelegate can return null. This fixes the typings by adding null to the return types.

Closes

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

🧢 Your Project:

[reidbarber]

Thanks for submitting! If you sign the CLA and close/re-open this it should build

[LFDanLu]

LGTM, thanks for the PR! We're are looking into making our own tsconfig strict in the future so hopefully we'll be more consistent with these null typings