[REQ] [Dart] Remove redundant null checks

[nickmeinhold]

Is your feature request related to a problem? Please describe.

The fromJson methods in my models have lots of redundant checks like this:

if (json['stop_distance'] == null) {
  stopDistance = null;
} else {
  stopDistance = json['stop_distance'];
}

Describe the solution you'd like

Remove redundant check and just assign:

stopDistance = json['stop_distance'];

Describe alternatives you've considered

I'll start a PR now, should just be a small change to class.mustache I believe.