Update Dart Model Generation Command to Support Optional Fields

[bhanuka96]

I have been using the following command to generate Dart model classes locally:

npm start -- "--lang dart --null-safety --final-props --copy-with -o jsons/sample.dart jsons/sample.json"

While the command works well, I noticed that all generated fields are marked as required, even when I haven't explicitly specified them as required arguments.

Expected Behavior:
The command should generate Dart model classes with optional fields for those properties that are not marked as required in the JSON schema.