Avoid empty cpu_info field

Change-Id: I7edc1e98ec219df143747dcb369dedb0fcbea279
Related-PROD: PROD-24578 (PROD:24578)
diff --git a/openstack/compute/v2/hypervisors/results.go b/openstack/compute/v2/hypervisors/results.go
index 46638cb..a72ed7e 100644
--- a/openstack/compute/v2/hypervisors/results.go
+++ b/openstack/compute/v2/hypervisors/results.go
@@ -112,10 +112,11 @@
 	default:
 		return fmt.Errorf("CPUInfo has unexpected type: %T", t)
 	}
-
-	err = json.Unmarshal(tmpb, &r.CPUInfo)
-	if err != nil {
-		return err
+	if len(tmpb) > 0 {
+		err = json.Unmarshal(tmpb, &r.CPUInfo)
+		if err != nil {
+			return err
+		}
 	}
 
 	// These fields may be passed in in scientific notation