fix: Change Dart Serializer for MapBuilder to be nullable

[jdk-21]

I tried using the generator cli for the Jellyfin OpenAPI spec in Dart, but when serializing a BuiltMap<String, String?> which had null as a value I got an error:

Error: Deserializing to 'DisplayPreferencesDto' failed due to: Deserializing to 'BuiltMap<String, String?>' failed due to: type 'Null' is not a subtype of type 'String' of 'value'

While investigating this issue I found this type mismatch. I don't have experience with this template language, but this should fix the issue.

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@jaumard @josh-burton @amondnet

---

Summary by cubic

Fix the Dart built_value serializer to emit nullable MapBuilder types for maps with nullable values, preventing errors when deserializing BuiltMap<String, T?>.

• Bug Fixes
  • Update serializers.mustache to use MapBuilder<String, {{dataType}}?> when isNullable is true, matching FullType.nullable(...); refresh the petstore_client_lib_fake sample to MapBuilder<String, JsonObject?> to confirm the fix in dio + built_value clients.

^{Written for commit 3e855e5. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

[wing328]

thanks for the pr

please follow step 3 to update the samples