[DART] Parsing null DateTime to JSON

Description

I'm not sure yet if this is a bug, or if I'm doing something wrong.

I've checked the dart/class.mustache file, and I think there lies the problem. When parsing a null DateTime property to json, it sets as an empty String. If I try to parse the generated json string, I get an exception when trying to DateTime.parse because the empty string is not an valid date.

Swagger-codegen version

2.4.2

Swagger declaration file content or url

{
    "type": "object",
    "properties": {
        "dateExample": {
            "type": "string",
            "format": "date-time",
            "description": "DateExample",
            "allowEmptyValue": false
        }
    },
    "title": "ExampleModel",
    "description": "Response model"
}

Command line used for generation

java -jar /swagger-codegen-cli.jar generate -i swagger.json -l dart -o sdk/dart -c deploy/swagger-generator.json -DbrowserClient=false

Steps to reproduce

1. Generate a dart client having a model with a date property
2. Parse the example model to json
3. Parse the given json back to the model. The error should occur now, as mentioned before, from a "DateTime.parse"

Related issues/PRs

Suggest a fix/enhancement

Change this line https://github.com/swagger-api/swagger-codegen/blob/f147928d82db0352761f93a7d947924a77160496/modules/swagger-codegen/src/main/resources/dart/class.mustache#L41

to:
{{#isDateTime}}'{{name}}': {{name}} == null ? null : {{name}}.toUtc().toIso8601String(){{^-last}},{{/-last}}{{/isDateTime}}{{^isDateTime}}'{{name}}': {{name}}{{^-last}},{{/-last}}{{/isDateTime}}

Start with modules/swagger-codegen/src/main/resources/dart/class.mustache at the referenced line and generate a Dart client using the provided command and schema. Reproduce the null DateTime serialization and round-trip described in the issue; done means the generated JSON preserves a null date-time value and can be parsed back without a DateTime.parse exception.