Possible NullPointerException in InstanceActionHandler

[jordanly]

After introducing the 'slaAware' field in this commit, there has been a possible NullPointerException in this line. This happens when an update only kills instances. There is no desiredState, so getDesiredState().getTask() will try to act on a null object. Other places in the code guard against this case by checking isSetDesiredState() before retrieval.

We should properly handle nulls in this case, and add generic end to end tests for only adding instances and only removing instances with updates.