controller: fix handling for invalid node name (#20)

If the model PVC is already in a bound state but the node hosting it
has been taken offline, the previous delete volume process would trust
the node IP marked on the PV, but since the node no longer exists,
gRPC request sent via that node IP would fail, and then preventing the
PV from being deleted.

This fix ensures that during each delete volume operation, we check the
node status, if the node no longer exists, we return success directly,
allowing the orphaned PV to be cleaned up properly.

Signed-off-by: imeoer <yansong.ys@antgroup.com>