snapshots_serialized(props=["createtxg"], ...) misformats the data

[asomers]

If you use snapshots_serialized and the only property in the list is createtxg, py-libzfs will not fetch it correctly. Instead of the usual parsed, source, rawvalue, value dict, it will simply return 0. However, if there are any other properties in the list, then all will be fetched correctly. From my spot-checking, only createtxg exhibits this behavior.

Environment

FreeBSD 13.1-RELEASE amd64, py-libzfs-1.1.2023020200. The bug must've been introduced sometime in the revision range 077cf6d..1a99a45.

Steps to Reproduce

Ensure that there is a dataset named "tank" with at least one snapshot. Then run this script:

from pprint import pprint
import libzfs

zfs = libzfs.ZFS()
badsnaps = zfs.snapshots_serialized(
    props=["createtxg"],
    datasets=["tank"],
    recursive=False
)

goodsnaps = zfs.snapshots_serialized(
    props=["used", "createtxg"],
    datasets=["tank"],
    recursive=False
)

print("Bad version of the snapshot list:")
pprint(badsnaps[0])
print("\nGood version of the snapshot list:")
pprint(goodsnaps[0])

The output will look like this:

Bad version of the snapshot list:
{'createtxg': '0',                                                   <--- Incorrect value
 'dataset': 'tank',
 'id': 'tank@zrepl_20230119_070154_000',
 'name': 'tank@zrepl_20230119_070154_000',
 'pool': 'tank',
 'snapshot_name': 'zrepl_20230119_070154_000',
 'type': 'SNAPSHOT'}                                                 <--- Dict ends with no 'properties' section

Good version of the snapshot list:
{'createtxg': '53209141',                                            <--- Correct value
 'dataset': 'tank',
 'id': 'tank@zrepl_20230119_070154_000',
 'name': 'tank@zrepl_20230119_070154_000',
 'pool': 'tank',
 'properties': {'createtxg': {'parsed': '53209141',                  <--- Expected 'properties' section
                              'rawvalue': '53209141',
                              'source': 'NONE',
                              'value': '53209141'},
                'used': {'parsed': 0,
                         'rawvalue': '0',
                         'source': 'NONE',
                         'value': '0B'}},
 'snapshot_name': 'zrepl_20230119_070154_000',
 'type': 'SNAPSHOT'}