[BUG][Go] undeclared name: isNil

Description

When generating Go structs with the latest docker container, a function isNil is used in the code but not generated.

openapi-generator version

v6.2.1

latest docker container (IMAGE ID: 0f04bb22fe23)

OpenAPI declaration file content or URL

Every file depends on this function.

Generation Details

func (o *MyObject) GetField() string {
	if o == nil || isNil(o.Field) {
		var ret string
		return ret
	}
	return *o.Field
}

Note that isNil(o.Field)

Steps to reproduce

$ docker run --rm -v \"${PWD}:/local\" \
	openapitools/openapi-generator-cli:latest  generate \
	-i openapi.yaml \
	-g go -o ${PWD}/gen/openapi \
	--global-property models,modelDocs=false \
	--additional-properties packageName=openapi"

Suggest a fix

Seems to be related to this MR: https://github.com/OpenAPITools/openapi-generator/pull/13843/files#diff-f85b9f51529f9922081dd92244891f831a26e126f4e27dd53bdaa03c7d55456c

My guess would be that because I only generate the models, this "utils" is not generated. So a fix would be to generate this utils anyway, even if only models are to be generated.

Run the provided Docker generation command with the models-only global property and confirm the undeclared isNil error. Then inspect the Go generator's model-only path and the utils generation change referenced in pull request #13843; done means generated models compile with isNil available.