Core: Support null and undefined in URL args

[ghengeveld]

Issue: -

What I did

This adds support for null and undefined arg values, so that removing an object property or array value in a JSON value will get properly synced to/from the URL.

Additionally, this fixes encoding of the space character as + rather than %20.

How to test

• Is this testable with Jest or Chromatic screenshots? no
• Does this need a new example in the kitchen sink apps? no
• Does this need an update to the documentation? no

If your answer is yes to any of these, please make sure to include it in your PR.

[coderkevin]

Just a few optional comments on this. In general, it looks good to me! 👍