Implement list of nullables with minimal refactorings

[nielsenko]

Allow list of nullable primitives in realm models, such as:

@RealmModel()
class _Player {
  @PrimaryKey()
  late String name;
  final scoresByRound = <int?>[]; // null means player didn't finish
}

Fixes #163 (supersedes #708)

[coveralls]

Pull Request Test Coverage Report for Build 3052154069

• -2 of 7 (71.43%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+5.7%) to 92.908%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
generator/lib/src/dart_type_ex.dart	3	5	60.0%

Totals
Change from base Build 3036688559:	5.7%
Covered Lines:	393
Relevant Lines:	423

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3125627969

• 5 of 7 (71.43%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.1%) to 86.602%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
generator/lib/src/dart_type_ex.dart	3	5	60.0%

Totals
Change from base Build 3115288468:	-1.1%
Covered Lines:	1797
Relevant Lines:	2075

---

💛 - Coveralls

[nirinchev]

Looks fine, even though I'd have preferred the const schema approach.

[blagoev]

Looks good, just want to make sure we have noticed the change in my comment.