Fix defaultValue for String schema in Dart

[ysawa]

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Description of the PR

Output defaultValue value for String is unquoted and it will cause syntax errors in Dart.

Markdown outputs is like:

# openapi.model.CharacterDetail

## Properties
Name | Type | Description | Notes
------------ | ------------- | ------------- | -------------
**name** | **String** |  | [default to null]
**description** | **String** |  | [optional] [default to ]

and, **description** | **String** | | [optional] [default to ""] may be better.

Also, generated Dart code is as below:

class CharacterDetail {

  String name = null;

  String description = ;

, and this should be String description = "";.

I hope it will be fixed.

@ircecho (2017/07) @swipesight (2018/09) @jaumard (2018/09)

[wing328]

@ysawa thanks for the fix, which looks good to me. If no further feedback or question, I'll merge it tomorrow (Thursday)

[ysawa]

@wing328 thanks for your review. I am looking forward to the merge.

[wing328]

@ysawa PR merged into master. Thanks for your contribution.

Do you mind filing the same fix for Dart Jaguar client generator: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/DartJaguarClientCodegen.java#L64 ?

cc @jaumard

[wing328]

@ysawa thanks again for the PR, which has been included in the v3.3.3 release: https://twitter.com/oas_generator/status/1062929948191510528