[Dart2] Fix compile errors for required enums without default value.

Created by: gilbsgilbs

Before:

  schemas:
    Order:
      type: object
      required: ["status"]
      properties:
        status:
          type: string
          enum:
            - placed
            - approved
            - delivered

Would produce the following Dart code which would fail to compile (because of a missing required parameter):

OrderEnum order = OrderEnum._();

This PR skips the enum initialization when no default value is provided.

Also, when a minimum value was specified for a property, I think there was a comment expected to display but wasn't (because it was in a wrong section). If fixed the comment value and put it in the proper section so that it shows.

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