[BUG][GO-SERVER] Enum fromValue functions always returns string in case of invalid values

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The current golang server enum template modules/openapi-generator/src/main/resources/go-server/model.mustache produces a bugged code for enum values.
The New{{{classname}}}FromValue function always returns an empty string when the ev is not valid, so for integer enums the code does not compile.

The bugged code is the following

func NewErrorCodesFromValue(v int32) (ErrorCodes, error) {
	ev := ErrorCodes(v)
	if ev.IsValid() {
		return ev, nil
	}

	return "", fmt.Errorf("invalid value '%v' for ErrorCodes: valid values are %v", v, AllowedErrorCodesEnumValues)
}

Expected output

func NewErrorCodesFromValue(v int32) (ErrorCodes, error) {
	ev := ErrorCodes(v)
	if ev.IsValid() {
		return ev, nil
	}
	noValue := new(ErrorCodes)
	return *noValue, fmt.Errorf("invalid value '%v' for ErrorCodes: valid values are %v", v, AllowedErrorCodesEnumValues)
}

openapi-generator version

The latest version is affected

OpenAPI declaration file content or url

Start with modules/openapi-generator/src/main/resources/go-server/model.mustache and generate a go-server project from the provided schema containing integer enum values. Check the generated New{{{classname}}}FromValue function and compile the result. Done means invalid integer enum values return the typed zero value and an error without compilation failures.