Block Editor: LinkControl: Align documentation with current behavior

[aduth]

This pull request seeks to address a few consistency issues with the LinkControl documentation:

• There is no className prop
• value is not required
• onChange will always receive an item, never null
• The onChange example referenced the wrong (old) prop onLinkChange, and the example used an impossible scenario that the changed item could be falsey.
• The tests reference behavior of "required props", but there are no required props

Separately, I would like to improve documentation and types concerning what a value actually is. These are intended as an incremental step in that direction.

Testing Instructions:

The only changes impact documentation and tests. There is no expected impact on the behavior of the application runtime.

Ensure unit tests pass:

npm run test-unit