handleDataMessage: Don't drop messages for empty GroupV1 groups

ts/models/messages.ts

@@ -2825,11 +2825,14 @@ export class MessageModel extends window.Backbone.Model<MessageAttributesType> {
       // We drop incoming messages for v1 groups we already know about, which we're not
       //   a part of, except for group updates. Because group v1 updates haven't been
       //   applied by this point.
+      // Note: if we have no information about a group at all, we will accept those
+      //   messages. We detect that via a missing 'members' field.
       if (
         type === 'incoming' &&
         !conversation.isPrivate() &&
         !isGroupV2 &&
         !isV1GroupUpdate &&
+        conversation.get('members') &&
         (conversation.get('left') || !conversation.hasMember(ourConversationId))
       ) {
         window.log.warn(