Update error handling for empty metrics responses in exporter

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

gateway/metrics/exporter.go

@@ -167,7 +167,9 @@ func (e *Exporter) getFunctions(endpointURL url.URL, namespace string) ([]types.
 		} else {
 			body = b
 		}
-	} else {
+	}
+
+	if len(body) == 0 {
 		return services, fmt.Errorf("no response body from /system/functions")
 	}
 
@@ -211,6 +213,10 @@ func (e *Exporter) getNamespaces(endpointURL url.URL) ([]string, error) {
 		}
 	}
 
+	if len(body) == 0 {
+		return namespaces, fmt.Errorf("no response body from /system/namespaces")
+	}
+
 	if err := json.Unmarshal(body, &namespaces); err != nil {
 		return namespaces, fmt.Errorf("error unmarshalling response: %s, error: %s", string(body), err)
 	}