Fixed issue where sub check would crash server if no subs existed

backend/subscriptions.js

@@ -214,6 +214,7 @@ exports.watchSubscriptionsInterval = async () => {
         if (!change) return;
         if (change['key'] === 'ytdl_subscriptions_check_interval' || change['key'] === 'ytdl_multi_user_mode') {
             current_sub_index = 0; // TODO: start after the last sub check
+            logger.verbose('Resetting sub check schedule due to config change');
             clearInterval(parent_interval);
             const new_interval = config_api.getConfigItem('ytdl_subscriptions_check_interval');
             parent_interval = setInterval(() => watchSubscriptions(), new_interval*1000);
@@ -224,6 +225,10 @@ exports.watchSubscriptionsInterval = async () => {
 
 async function watchSubscriptions() {
     const subscription_ids = await getValidSubscriptionsToCheck();
+    if (subscription_ids.length === 0) {
+        logger.info('Skipping subscription check as no valid subscriptions exist.');
+        return;
+    }
     checkSubscription(subscription_ids[current_sub_index]);
     current_sub_index = (current_sub_index + 1) % subscription_ids.length;
 }
@@ -243,7 +248,6 @@ async function checkSubscription(sub_id) {
     }
 
     await exports.getVideosForSub(sub.id);
-    // TODO: make sure if multi user mode changes we are also cancelling this
 }
 
 async function getValidSubscriptionsToCheck() {