If a user manually deletes a scaleset managed by OneFuzz, then `get_vmss_size` returns None.
When this happens, `Scaleset.shutdown` generates an exception from the `logging.info` call on line 573.
This PR handles this edge condition.
As is, when multiple tasks are running on a single node, if any one of them stops, the node gets reimaged.
This changes the behavior such that when a node with multiple tasks has one task stop, the other tasks will continue.
