Fix undefined error

Author: dnicolson

lib/switchbot.js

@@ -132,7 +132,9 @@ class Switchbot {
             for (let addr in peripherals) {
               device_list.push(peripherals[addr]);
             }
-            resolve(device_list);
+            if (device_list.length) {
+              resolve(device_list);
+            }
           };
 
           // Set a handler for the 'discover' event