Make nullable fix on all vars in Go (!8820) · Merge requests · OpenAPI Tools / openapi-generator

Created by: therve

This applies the Nullable change to all variables on the model, instead of just the ones in vars.

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Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
Run the following to build the project and update samples:
```
./mvnw clean package 
./bin/generate-samples.sh
./bin/utils/export_docs_generators.sh
```
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