ClassLoader::registerAsParallelCapable should only exceed if the caller is a subclass of ClassLoader.

If the caller is null, the method could simply return false. We could also consider to throw IllegalCallerException but it should probably throw IllegalCallerException for all other invalid cases such as a non-null caller is not a subclass of ClassLoader. It seems that returning false is a reasonable solution for simplicity

The existing implementation throws NPE since this method was added.