You can run any filesystem on a bare block device with or without partitions, however the danger is that both people and OSes expect to see a partition table on a block device to realize that there is data present.
(both raw disks and partitions are just block devices, after all)
ZFS actually creates such a guardian partition table which is why it's safe to do it on an unpartitioned device:
$ zpool status
...
NAME STATE READ WRITE CKSUM
storage ONLINE 0 0 0
raidz2-0 ONLINE 0 0 0
sda ONLINE 0 0 0
sdb ONLINE 0 0 0
...
$ sudo gdisk -l /dev/sda
...
MBR: protective
GPT: present
...
Number Start (sector) End (sector) Size Code Name
1 2048 7814019071 3.6 TiB BF01 zfs
9 7814019072 7814035455 8.0 MiB BF07