fix empty avatar being undefined rather than ''

building ClosedGroupUpdate message via ts makes it have an avatar
content with fields empty if we pass '' as avatar.

we wamt no avatar field at all

js/background.js

@@ -700,7 +700,7 @@
         return;
       }
 
-      const nullAvatar = '';
+      const nullAvatar = undefined;
       if (avatar) {
         // would get to download this file on each client in the group
         // and reference the local file
@@ -831,7 +831,7 @@
           recipients: allMembers,
           active: true,
           expireTimer: 0,
-          avatar: '',
+          avatar: undefined,
         },
         confirm: () => {},
       };

ts/session/messages/outgoing/content/data/group/ClosedGroupUpdateMessage.ts

@@ -43,6 +43,13 @@ export abstract class ClosedGroupUpdateMessage extends ClosedGroupMessage {
       throw new Error('admins has not the correct type');
     }
 
+    if (
+      params.avatar !== undefined &&
+      !(params.avatar instanceof SignalService.AttachmentPointer)
+    ) {
+      throw new Error('avatar has not the correct type');
+    }
+
     this.name = params.name;
     this.members = params.members;
     this.admins = params.admins;