API doc generation removes children when function is optional

[timroes]

There seems to be some edge-cases when generating the API docs, that are causing the children object in the output to vanish if the function itself is optional. See https://github.com/elastic/kibana/pull/107128/files for an example where this happened.

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[stacey-gammon]

Confirmed the bug with a test. Now to figure out why.

  ● interfaces and classes › Optional function on interface includes children

    expect(received).toBeDefined()

    Received: undefined

      430 |     expect(fn).toBeDefined();
      431 |     expect(fn?.type).toBe(TypeKind.FunctionKind);
    > 432 |     expect(fn?.children).toBeDefined();
          |                          ^
      433 |     expect(fn?.children?.length).toBe(1);
      434 |   });
      435 |