Related to #45910
When IndexNodeBinding mode is enabled, DataCoord skips session watching
for datanodes but the dnSessionWatcher field remains nil. This causes a
panic when other code attempts to access the watcher.
This fix introduces an EmptySessionWatcher as a placeholder for the
IndexNodeBinding mode scenario. The empty watcher implements the
SessionWatcher interface with no-op methods, preventing nil pointer
dereferences while maintaining the expected interface contract.
Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>
ea4bbbda3a