FreeBSD: Fix potential boot panic with bad label

[ghost]

Motivation and Context

https://forums.FreeBSD.org/threads/nvme-adapter-with-hp-ex900-ssd-pci-e-nvme.85816/post-590379
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256368
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=268116

vdev_geom_read_pool_label() can leave NULL in configs when it cannot read a label.
We then expect to be able to read the config and panic.

Description

Check for NULL and skip it when iterating through configs to generate rootconf.

How Has This Been Tested?

An early version of this fix was tested https://forums.FreeBSD.org/threads/nvme-adapter-with-hp-ex900-ssd-pci-e-nvme.85816/post-590379

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]

Needs fixing in 2.1, too.

[ghost]

• Rebased
• Check for the initialized best_txg value rather than the uninitialized best_cfg