Add defensive checks against undefined messages to _is_summarized_message.

Should fix the issue with empty realms.

(imported from commit afca70ebf3b7b74ae0d0c269c72b4f8d54fc254b)

static/js/message_list.js

@@ -227,7 +227,8 @@ MessageList.prototype = {
     },
 
     _is_summarized_message: function (message) {
-        if (message.flags === undefined) {
+        if (!feature_flags.summarize_read_while_narrowed ||
+            message === undefined || message.flags === undefined) {
             return false;
         }
         if (this.summarize_read === 'home') {