The Rules DSL follows the standard Java order of operations and || should be succeed fast (i.e. if the first argument is true then the rest of the comparisons are skipped). Similarly, && should be fail fast (i.e. if the first argument is false the rest of the comparisons are skipped.
There is a special State Object called NULL and this is what you are comparing the Item’s state to. This special state used to be called Undefined and the change to NULL is one of the changes introduced in 2.0. I, like you, feel NULL is more confusing than enlightening and would have rather it stayed named Undefined.
Have you looked into the Expire binding? You can configure your Item to automatically be set to NULL (or any other state via an update or command) after it hasn’t been updated for x minutes. For example:
Number ZWaveMultiSensor_SensorTemperature ... { ..., expire="Xm" }
Replace “X” with your minutes. After that amount of time without an update to the Item it will be set to NULL. No need for a rule.
Beyond that you can just use:
ZWaveMultiSensor_SensorTemperature.postUpdate(NULL)
in your existing rule.
See the second example in the Generic Is Alive Deisgn Pattern for a more fully implemented example.