Handle gnt-instance shutdown --all for empty clusters

The current code gives:
Failure: prerequisites not met for this operation:
error type: wrong_input, error details:
Selection filter does not match any instances

Signed-off-by: Stephen Shirley <diamond@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>