Consider changing the approach to null / undefined in generated TS wrappers

[vlukashov]

Originally, all generated TS types did not allow null / undefined values. Later(see https://github.com/vaadin/vaadin-connect/issues/363), all null / undefined values were explicitly allowed in all generated types (the current state). That has raised a number of DX concerns. The task of this ticket is to re-consider the approach and see if some aspects of the DX can be improved without hurting the type safety.

DX concerns with the current approach to null / undefined values:

• confusion between null and undefined in TS:

  Is this something you would expect to work?

  @customElement("my-app")
  export class MyApp extends LitElement {
    measurement?: Measurement;
  
    async connectedCallback() {
      super.connectedCallback();
      this.measurement = await Service.getMeasurement("temperature");
    }
  }

  spoiler, it does not

  Type 'Measurement | null' is not assignable to type 'Measurement | undefined'.
  Type 'null' is not assignable to type 'Measurement | undefined'.ts(2322)

• complex type definitions in the generated TS code. Some details in the review comment to https://github.com/vaadin/vaadin-connect/pull/369

  • collections: null / empty / can have null values / cannot have null values

    Can I even annotate that a List<Person> cannot contain null values?

  (this particular case is addressed by a separate ticket: https://github.com/vaadin/vaadin-connect/issues/388)