fix: filter already-deleted sessions when deleting a group

deleteSession uses setSub(..., undefined) to remove session references
from group2sessions and author2sessions, but this can leave null entries
in the sessionIDs object. When deleteGroup later iterates Object.keys
of sessionIDs and calls deleteSession on each, it throws "sessionID
does not exist" for the already-deleted sessions.

Now deleteGroup filters out null/falsy session entries before attempting
to delete them.

Fixes: #5798

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: JohnMcLear

src/node/db/GroupManager.ts

@@ -59,8 +59,10 @@ exports.deleteGroup = async (groupID: string): Promise<void> => {
 
   // Delete associated sessions in parallel. This should be done before deleting the group2sessions
   // record because deleting a session updates the group2sessions record.
+  // Filter out already-deleted sessions: deleteSession sets the value to null/undefined via setSub,
+  // but the key remains in the object. See https://github.com/ether/etherpad-lite/issues/5798
   const {sessionIDs = {}} = await db.get(`group2sessions:${groupID}`) || {};
-  await Promise.all(Object.keys(sessionIDs).map(async (sessionId) => {
+  await Promise.all(Object.keys(sessionIDs).filter((id) => sessionIDs[id]).map(async (sessionId) => {
     await sessionManager.deleteSession(sessionId);
   }));