The generated code for interfaces includes default implementations for
optional properties (since adding a new optional property is a non
breaking change in TypeScript, this allows us to preserve this property
in Java, too).

However, if two distinct super-interfaces declare the same property, the
child inherits two unrelated default implementations for the same
method, which results in dispatch ambiguity that is illegal.

In order to remove the ambiguity, a new default implementation must be
provided on the child interface, so that the dispatch is unambiguous
again.

Fixes #22556

Co-authored-by: Niranjan Jayakar <nija@amazon.com>