Make plugin init a no-op if an URL is not provided

Ideally, the HLS plugin would be able to seamlessly participate in source selection but that isn't implemented yet. Until then, require an explicit manifest URL in the init options and do nothing if it's missing.

@@ -31,8 +31,11 @@ var
 
     if (typeof options === 'string') {
       url = options;
-    } else {
+    } else if (options) {
       url = options.url;
+    } else {
+      // do nothing until the plugin is initialized with a valid URL
+      return;
     }
 
     // expose the HLS plugin state