Add null support for data property of InputEvent

[kobijones111529]

Description of the change

Change InputEvent data_ FFI import to private _data_ function returning type Nullable String to accommodate null data property. Export data_ function, now returning type Maybe String.

---

Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[garyb]

Thanks! Sorry about the ridiculously long delay before merging.

[kobijones111529]

Thanks! Sorry about the ridiculously long delay before merging.

No worries! Happy to make a contribution :)