[GO] Fix exception in constructExampleCode with oneOf primitive type

[okhowang]

close (#12357)

PR checklist

• Read the contribution guidelines.
• 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
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.
  @antihax @grokify @kemokemo @jirikuncar @ph4r5h4d

[wing328]

Tested locally to confirm the fix:

[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/docs/DefaultApi.md
[main] INFO  o.o.codegen.utils.URLPathUtils - 'host' (OAS 2.0) or 'servers' (OAS 3.0) not defined in the spec. Default to [http://localhost] for server URL [http://localhost/]
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/api/openapi.yaml
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/README.md
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/git_push.sh
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/.gitignore
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/configuration.go
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/client.go
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/response.go
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/go.mod
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/go.sum
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/.travis.yml
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/utils.go
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/.openapi-generator-ignore
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/.openapi-generator/VERSION
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/gooneof/.openapi-generator/FILES
################################################################################
# Thanks for using OpenAPI Generator.                                          #
# Please consider donation to help us maintain this project 🙏                 #
# https://opencollective.com/openapi_generator/donate                          #
################################################################################
➜  openapi-generator git:(okhowang-fix/go-one-of-exception) ✗ 

Thanks for the PR.

[wing328]

Thanks again for the PR, which has been included in the v6.0.0 release: https://twitter.com/oas_generator/status/1529669733934841856