Resource description for platform mbean properties that throw UnsupportedOperationException should say nillable="true"

Some platform mbean getters are documented to throw a UOE on some VMs. The read-resource handler will catch the UOE and leave the attribute undefined, but the description says it's not nillable.

Specifically, PlatformMBeanDescriptions.getThreadingResource()'s THREAD_CPU_TIME_ENABLED attribute, although there may well be others.

This leads to this unit test failure on jvms where the UOE is thrown:

failure message="thread-cpu-time-enabled is undefined"
type="junit.framework.AssertionFailedError">junit.fra mework.AssertionFailedError: thread-cpu-time-enabled is undefined
at junit.framework.Assert.fail(Assert.java:50)
at junit.framework.Assert.assertTrue(Assert.java:20)
at
org.jboss.as.platform.mbean.PlatformMBeanResourceUnitTestCase.validateResource(PlatformMBeanResourceUn
itTestCase.java:595)
at org.jboss.as.platform.mbean.PlatformMBeanResourceUnitTestCase.basicResourceTest(PlatformMBeanResourceUnitTestCase.java:563)
at
org.jboss.as.platform.mbean.PlatformMBeanResourceUnitTestCase.testThreadingMXBean(PlatformMBeanResourceUnitTestCase.java:340)