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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• Read the contribution guidelines.
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• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
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