ParentFirstClassLoader returns Null if class is not found

The ParentFirstClassLoader returns a null if the class is not found. This is in violation of the ClassLoader contract - it should throw a ClassNotFoundException.

One side effect of this is an attempt to deploy an MBean into the JMX environment using MX4J results in a NullPointerException as MX4J attempts to look for the MBean descriptor class. MX4J uses the classloader that loaded the MBean and expects the ClassNotFound exception to be thrown if the class does not exist.