zed: Fix zed ASSERT on slot power cycle

We would see zed assert on one of our systems if we powered off a
slot. Further examination showed zfs_retire_recv() was reporting
a GUID of 0, which in turn would return a NULL nvlist. Add
in a check for a zero GUID.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #15084