Fix defaultValue for String schema in Dart

Created by: ysawa

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

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)