[BUGFIX][dart-dio] Align enum and enum_inline generation result

[vasilich6107]

Align enum and enum_inline generation result

Previously the PR(#15387) updated enum generation but forgot to change the enum_inline template

PR checklist

• Read the contribution guidelines.
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  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

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  Commit all changed files.
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[vasilich6107]

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

Also @wing328 could you pay a little bit of attention.
This PR just aligns the implementation so should be easy to review.

[ahmednfwela]

Is isNull check necessary ?

[vasilich6107]

OpenApiSchema specifies a nullable enum to be defined including null as one of the possible values. https://swagger.io/docs/specification/data-models/enums/

We do not want thins null to be present among other items

This is why i'm filtering it out

[ahmednfwela]

LGTM @wing328

[vasilich6107]

Hi @wing328
All fixed and approved, could you merge

[vasilich6107]

@4brunu could you help with merging the PR?

[wing328]

@vasilich6107 thanks for the PR, which has been merged.