Avoid MonitorObjectCollection to grow in size (#2169)

Currently the published objects are inserted in the
`MonitorObjectCollection` via the `TObjArray::Add()` method,
which always appends new elements at the end of the array.

When un-publishing objects, they are removed via the
`TObjArray::Remove()` method, which does not reduce the size of the
TObjArray. Instead, the removed slots are set to `nullptr`.

The proposed solution is to add a call to `TObjArray::Compress()`
after `TObjArray::Remove()`, which forces the removal of empty
slots from the array.

Author: aferrero2707

Framework/src/ObjectsManager.cxx

@@ -131,6 +131,7 @@ void ObjectsManager::stopPublishing(const string& objectName)
 {
   auto* mo = dynamic_cast<MonitorObject*>(getMonitorObject(objectName));
   mMonitorObjects->Remove(mo);
+  mMonitorObjects->Compress();
 }
 
 void ObjectsManager::stopPublishingAll()