JmxTool should support querying all objects when object-name is omitted

One usecase of JmxTool is to allow user to query all existing objects and attributes, such that the developer can quickly lookup the right object name and attribute name in the output. Indeed, existing API of JmxTool allows user to do this when object-name is not specified in argument.

However, current implementation of JmxTool doesn't support this operation due to the way it is implemented. This is partly due to the fact that some JMX object, such as java.lang:type=MemoryPool, doesn't have value for all its attributes.

The patch submitted here will fix the problem.