[Dart] Codegen fails with any type {}

Description

When an object is defined with a property with no type definition, the codegen reports, "Could not process model '{Object-Name}'. Please make sure that your schema is correct!"

Swagger-codegen version

Using 2.4.0-SNAPSHOT

Swagger declaration file content or url

"Notification":{
  "type":"object",
  "properties":{
    "id":{"type":"integer","format":"int64"},
    "message":{"type":"string"},
    "icon_url":{"type":"string"},
    "metadata":{},
    "read_status":{"type":"boolean"},
    "created_at":{"type":"string","format":"date-time"},
    "updated_at":{"type":"string","format":"date-time","x-nullable":true}
  },
  "additionalProperties":false,
  "required":[
    "id",
    "message",
    "icon_url",
    "metadata",
    "read_status",
    "created_at",
    "updated_at"
  ]
}

Note the metadata property is defined with any type.

Command line used for generation

java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -i ~/Downloads/medanswers_swagger.json -l dart -o ~/swagger-output/dart/

Steps to reproduce

• Modify a property on an object in the pet store example to have any type {}.
• Try to generate the code against the modified pet store example using the codegen CLI.
• Codegen will fail with the stated error.

Related issues/PRs

This appears to have been introduced with the changes to the Dart codegen for compatibility with flutter. Codegen does not fail with version 2.3.1.

https://github.com/swagger-api/swagger-codegen/pull/7418

Suggest a fix/enhancement

I'm not familiar enough with Mustache syntax to suggest a fix, but I suspect it will require a modification to https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/dart/class.mustache

Start with the Dart generation path and the referenced modules/swagger-codegen/src/main/resources/dart/class.mustache template, then reproduce the failure using the modified pet store example and the CLI command shown. Done means Dart code generation succeeds when a model property is defined as an empty object, without regressing the existing example.