fix: handle nil servers in InstanceV2 #398 (#402)

We did not properly handle nil pointers returned from `lookupServer`.

Based on the current usage in `k/cloud-provider` the methods are called
in these contexts:

- `InstanceShutdown` is only called for instances that actually do exist
(as verified by a prior call to `InstanceExists`)
- `InstanceMetadata` is called without verifying that the instance
exists, one call site accepts a nil InstanceMetadata, but the other does
not.
Closes #398