Migration to null safety dart 2.0

[Kalela]

I migrated all the files to their null safe versions by running dart migrate and removing some of the bugs that came up. Here are the changes that I made:

1. Some factory constructors had to be converted to static methods. This is because null safety Dart 2.0 allows static methods to return null but not factory methods.
2. Updated pubspec dependencies to null safe variants.
3. Made most fields nullable and maintained the null checks that were present before. Especially in factory constructors. I did not want to start changing the logic for where the return values were used.

Closes #49

[niravanu]

@witochandra Could you accept this pull request. Thanks!

[witochandra]

Thanks @Kalela for this PR. 👍

Did few refactoring after this PR, basically bringing back factory methods so it is consistent.