Icon wrapper should handle null or undefined "icon" prop gracefully

If we pass null into <Icon icon={null} /> the whole render pipeline fails.

This leads to broken builds and makes mocking non-existing Icons at least tedious.

I lost about 2h because of this in different projects after updating npm & ui-components not realizing which Icons were not loaded.

Please consider falsy values and display a placeholder instead.

Maybe important that this gets to ui-icons too?

ping @apittori @d.thill @dan