upstream property
Upstream credits the third-party work this project was published from, and is settable after the fact because the live demos are the ones that most need crediting. An explicit empty string CLEARS the credit; absent leaves it.
Please note: This property should have been non-nullable! Since the specification file does not include a default value (using the "default:" property), however, the generated source code must fall back to having a nullable type. Consider adding a "default:" property in the specification file to hide this note.
Implementation
String? upstream;