[DART] Add null-safety for dart 2.12+

Created by: harsimranmaan

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

Starting with DART 2.12 dart packages support sound null-safety making it easy to detect common pitfalls during static analysis.

The current implementation of DART2 does not follow null-safety principles.

Describe the solution you'd like

The generated code should follow sound null-safety as per https://dart.dev/null-safety.

If an openapi field is not marked as required, it should be nullable by default: eg:

    Customer:
      type: object
      required:
        - name
        - tenant_id
      properties:
        id:
          type: string
          format: uuid
          readOnly: true
        created_at:
          type: string
          format: date-time
          readOnly: true
        updated_at:
          type: string
          format: date-time
          readOnly: true
        created_by:
          type: string
          format: uuid
          readOnly: true
        tenant_id:
          type: string
          format: uuid
        name:
          type: string
          minLength: 3
          maxLength: 50

should generate a class Customer with fields like String? id, String tenantId, String name, Datetime? createdAt ...

Describe alternatives you've considered

Use the legacy mode until null-safety is supported.

Additional context

Required by https://github.com/gibahjoe/openapi-generator-dart/issues/26