zed: Avoid core dump if wholedisk property does not exist

[ixhamza]

Motivation and Context

zed aborts and dumps core in vdev_whole_disk_from_config() if the wholedisk property does not exist. make_leaf_vdev() adds the property but there may be already pools that don't have the wholedisk in the label.
OpenZFS PR: openzfs#14062

How Has This Been Tested?

By creating pools through py-libzfs that did not populate the wholedisk parameter previously.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[ghost]

Ok to get this in for the freeze, but please also open a PR upstream for the zfs-2.1.7-staging branch.