JsonSchemaContent: Minor clean up
- Double line break.
- Allow subclasses to change $modelId. EventLogging depends on MediaWiki's JsonSchema to be extendable, seems fair for it to allow extension as well.
- Make methods explicitly 'public' to match the parent class.
- Minor documentation tweaks.
- Follows-up I335ad42bb8: Simplfy objectRow even further by special casing one key and calling the parent otherwise.
- Fix "PHP Notice: Undefined offset: 1" followed by "PHP Fatal error: Call to a member function getTitle() on null" in objectRow. Trigger when user input for a '$ref' property does not contain a slash.