Add const constructor to None and getOrNull to Option

[eEQK]

This PR consists of 2 commits:

1. Add const constructor to None

Allows for specifying None as default constructor parameter:

class Foo {
  Foo({
    Option<String> param = const None(),
  });
}

2. Add getOrNull method to Option

This one's pretty self-explanatory, as seen in tests:

/// getOrNull
expect(a.getOrNull(), null);
expect(b.getOrNull(), 2);

[eEQK]

Hey @cbyad, is there anything missing? any chance you could merge or reject this PR? Thanks in advance 🥰