* Fix nullability of Children extension
* Fix nullability of methods throughout the CMS
* Fix return types of some methods that cannot return null
* Revert nullable changes to result of ConvertSourceToIntermediate for property editors (whilst some property editors we know won't return null, it seems more consistent to adhere to the base class and interface nullability definition).
* Updated new webhook events to align with new nullability definitions.
* Reverted content editing service updates to align with base classes.
* Applied collection nullability updates on content repository to interface.
* Reverted value converter updates to match interface.
* Applied further collection updates to interface.
* Aligned media service interface with implementation for nullability.
* Update from code review.
---------
Co-authored-by: Ivo van der Bruggen <ivo@dutchbreeze.com>
Co-authored-by: Ivo van der Bruggen <ivo@vdbruggensoftware.com>
Co-authored-by: Andy Butland <abutland73@gmail.com>
c3e93f143c