* Issue 1766 Modified mustache files for Go to support nullable in the spec v3.0+; Updated model files running .sh scripts for Go.
* Add "nullable" to fake yaml
* Add sample script for OAS3
* Fix output folder (openapi3)
* Run bin/openapi3/go-petstore.sh
* Update samples
* Update jaxrs-jersey
* Update python and php samples
* Add bin/openapi3/go-gin-petstore-server.sh
* Run bin/openapi3/go-gin-petstore-server.sh
* Update bin/openapi3/go-petstore-server.sh to generate "nullable" samples
* Run bin/openapi3/go-petstore-server.sh
* Fix duplicated `import`
