LUVerifyCluster: Fix bug with offline nodes

[…]
 * Other Notes
   - NOTICE: 1 offline node(s) found.
 * Hooks Results
Failure: command execution error:
iteration over non-sequence

Commit a0c9776a introduced an error simulation mode to LUVerifyCluster.
Due to a small mistake, offline nodes weren't skipped when checking the
results of verification hooks and iterating over None raises an
“iteration over non-sequence” error.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>