[Go] Fix generated client tests when there is no response body

Created by: ctreatma

Closes #14079 (closed)

Currently, the Go client generator produces tests that are incorrect when an API endpoint response does not include a response body. This updates the api_test template to remove the expectation for a response body if the API endpoint under test does not produce a response body. A unit test is also added to the Go generator to validate that the api_test file is generated correctly for those API endpoints.

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.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
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