[FIX] mail: no crash on call from another device

Before this commit, when in a call on another device
and then calling from another device, there might be
the following traceback:

```
TypeError: Cannot read properties of undefined (reading 'lastObservedRecords')
at ModelManager.removeListener
at RelationSet.delete
at RelationSet.clear
at ModelField._setRelationReplace
at ModelField.parseAndExecuteCommands
at ModelManager._update
at Listener.onChange
at ModelManager._notifyListenersInUpdateCycle
at ModelManager._flushUpdateCycle
at ModelManager.update
```

This is caused by removing a non-existent listener on a field, very
likely related to a relational field from the call being terminated.

This commit fixes the issue by silently skipping removal of listener
when it's undefined.

Task-3241562

closes odoo/odoo#116192

X-original-commit: 24af22187ce8401d925edb1dda02652f993c7394
Signed-off-by: Louis Wicket (wil) <wil@odoo.com>

addons/im_livechat/static/src/legacy/model/model_manager.js

@@ -311,6 +311,9 @@ export class ModelManager {
      * @param {Listener} listener
      */
     removeListener(listener) {
+        if (!listener) {
+            return;
+        }
         this._listeners.delete(listener);
         this._listenersToNotifyInUpdateCycle.delete(listener);
         this._listenersToNotifyAfterUpdateCycle.delete(listener);