image: remove EINA_ARG_NONNULL check for parameter

image: remove EINA_ARG_NONNULL check for parameter

   in evas_object_image_file_get(const Evas_Object *obj,
                              const char **file,
                              const char **key)
   remove check for second parameter "file", because it
   contradicts comment's statement, that NULL can be passed,
   if parameter not needed.
   All needed NULL checks of parameter are present inside func.