Issue 1766 Modified mustache files for Go to support nullable in the …

Created by: alex-korobko

…spec v3.0+; Updated model files running .sh scripts for Go.

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Updated mustache files to enable the nullable support (https://github.com/OpenAPITools/openapi-generator/issues/1766) Ran all ./bin/go*.sh files to update the models; Not sure though how to test the changes though/ The integration test (as it recommended in the guide) classes for the Golang are using specs from the ver 2.0, so it seems like there is no reason to use them. Should we try to make new classes for that? Do you have any guide for that?

@antihax @bvwells @grokify @kemokemo