Fix GCP instance status mapping

We've found that occasionally instance data received from GCP API holds an empty status value. While debugging we've found that it happens while Creating or Deleting action on the instance is in place.

The explanation may be that the instance is either already added to the instance group list (so visible in the API) but the PROVISIONING status was not yet set on it. Or that it's almost done deleting (and already have lost the DELETING status) but was not yet removed from the list when API request was made.

This MR is fixing this specific case.