feat: assign random user name when not set (#6663)

src/excalidraw-app/collab/Collab.tsx

@@ -380,6 +380,13 @@ class Collab extends PureComponent<Props, CollabState> {
   startCollaboration = async (
     existingRoomLinkData: null | { roomId: string; roomKey: string },
   ): Promise<ImportedDataState | null> => {
+    if (!this.state.username) {
+      import("@excalidraw/random-username").then(({ getRandomUsername }) => {
+        const username = getRandomUsername();
+        this.onUsernameChange(username);
+      });
+    }
+
     if (this.portal.socket) {
       return null;
     }