Gets information about the property that this method call is for, if any.
This will only be set in the case of an invocation in response to a property Get or Set call that has been directed to the method call 
            handler for an object on account of its property_get or property_set vtable pointers being unset.
See DBusInterfaceVTable for more information.
If the call was GetAll, null will be returned.
| this | 
| a DBusPropertyInfo or null |