[bug][dart] Fix enums with default value

[kuhnroyal]

• fix enum property with default value not being correctly generated
• simplify template expression (I know its now more complex due to the enum handling but there is only one branch for @required and this.name)

PR checklist

• Read the contribution guidelines.
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CC @swipesight (2018/09) @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12)

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[wing328]

CircleCI failure already fixed in the master.