Fix ErrorTemplate for variants with empty fields

[bancek]

I've tested this generator with my 800 line UDL file and there was just one bug. Everything else worked without any problems.

If there is a variant without fields for an interface error, the template adds an extra comma.

Example UDL:

[Error]
interface ValidationError {
  InvalidUser(i32 user_id);
  InvalidMessage(string message);
  InvalidUserAndMessage(i32 user_id, string message);
  UnknownError();
};

Generated code (before this fix):

func (err ValidationErrorUnknownError) Error() string {
    return fmt.Sprint("UnknownError",
        ,
    )
}

Thanks for this awesome generator.