Data: Allow undefined return from withSelect mapSelectToProps

[aduth]

This pull request seeks to allow a falsey return value from a withSelect mapping function mapSelectToProps . This is a divergence from React Redux which enforces that this value be an object, leading to some awkward workarounds to ensure an object is returned. With these changes, a falsey value is assumed as equivalent to an empty object of props.

Further, the changes here resolve an issue where props may linger after a subsequent mapping call returns a different set of object keys. Previously we used a component's setState to update mapped props, but this behaves as a patch application. Previously React supported a this.replaceState method (deprecated in React 0.13) for this purpose, but it's easy enough to replicate with choosing an arbitrary key in state to apply the mapped value to.

Testing instructions:

Ensure unit tests pass:

npm run test-unit

[aduth]

Noting that this behavior was anticipated as part of the documentation updates in #5379:

Specific implementation differences from Redux and React Redux:

[...]

• In React Redux, a mapStateToProps function must return an object.
  • In @wordpress/data, a withSelect mapping function can return undefined if it has no props to inject.