ARM client: survive empty response and error

We're seeing legacy-cloud-providersazure/clients
(in our case, the synchronized copy used by cluster-autoscaler)
segfaulting under heavy pressure and ARM throttling, like so:

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x1b595b5]
cluster-autoscaler-all-79b9478bf5-cgkg8 cluster-autoscaler
goroutine 82 [running]:
k8s.io/autoscaler/cluster-autoscaler/vendor/k8s.io/legacy-cloud-providers/azure/clients/armclient.(*Client).Send(0xc00052f520, 0x3b6dc40, 0xc000937440, 0xc000933f00, 0x0, 0x0)
     /home/jb/go/src/k8s.io/autoscaler/cluster-autoscaler/vendor/k8s.io/legacy-cloud-providers/azure/clients/armclient/azure_armclient.go:122 +0xb5
```

Reason is the ARM client expects `sendRequest()` to return either
a non nil *retry.Error, or a non nil *http.Response, while both
can be nil.

Author: bpineau

staging/src/k8s.io/legacy-cloud-providers/azure/clients/armclient/azure_armclient.go

@@ -109,6 +109,11 @@ func (c *Client) sendRequest(ctx context.Context, request *http.Request) (*http.
 		request,
 		retry.DoExponentialBackoffRetry(&sendBackoff),
 	)
+
+	if response == nil && err == nil {
+		return response, retry.NewError(false, fmt.Errorf("Empty response and no HTTP code"))
+	}
+
 	return response, retry.GetError(response, err)
 }