if a zone without and host, cannot display in 'show zones'

[HarrisChu]

general speaking, as we can drop hosts from zone, and make a zone without any hosts, it should more verification for that.
e.g.

1. add zone to group, then drop all host in zone, then create space.
2. drop all hosts in zone, then add the zone to a group, then create space.

(root@nebula) [(none)]> add zone z1 "lekesr-storaged-0.lekesr-storaged-headless.default.svc.cluster.local":9779,"lekesr-storaged-1.lekesr-storaged-headless.default.svc.cluster.local":9779
Execution succeeded (time spent 2881/3852 us)

Thu, 23 Sep 2021 17:21:04 CST

(root@nebula) [(none)]>
(root@nebula) [(none)]>
(root@nebula) [(none)]> show zones
+------+------------------------------------------------------------------------+------+
| Name | Host                                                                   | Port |
+------+------------------------------------------------------------------------+------+
| "z1" | "lekesr-storaged-0.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
| "z1" | "lekesr-storaged-1.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
| "z2" | "lekesr-storaged-2.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
| "z3" | "lekesr-storaged-3.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
| "z4" | "lekesr-storaged-4.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
| "z4" | "lekesr-storaged-5.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
Got 6 rows (time spent 1961/2975 us)

(root@nebula) [(none)]> drop host "lekesr-storaged-0.lekesr-storaged-headless.default.svc.cluster.local":9779 from zone z1
Execution succeeded (time spent 1469/2004 us)

(root@nebula) [(none)]> drop host "lekesr-storaged-1.lekesr-storaged-headless.default.svc.cluster.local":9779 from zone z1

Execution succeeded (time spent 1952/2600 us)

(root@nebula) [(none)]> show zones
+------+------------------------------------------------------------------------+------+
| Name | Host                                                                   | Port |
+------+------------------------------------------------------------------------+------+
| "z2" | "lekesr-storaged-2.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
| "z3" | "lekesr-storaged-3.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
| "z4" | "lekesr-storaged-4.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
| "z4" | "lekesr-storaged-5.lekesr-storaged-headless.default.svc.cluster.local" | 9779 |
+------+------------------------------------------------------------------------+------+
Got 4 rows (time spent 1145/1794 us)

Thu, 23 Sep 2021 17:22:57 CST

(root@nebula) [(none)]>       ADD GROUP g1 z1

Execution succeeded (time spent 1649/2619 us)

Thu, 23 Sep 2021 17:25:56 CST

(root@nebula) [(none)]>
(root@nebula) [(none)]>
(root@nebula) [(none)]> show groups
+------+------+
| Name | Zone |
+------+------+
| "g1" | "z1" |
+------+------+
| "g2" | "z2" |
+------+------+
| "g2" | "z3" |
+------+------+
| "g2" | "z4" |
+------+------+
Got 4 rows (time spent 1550/2325 us)

(root@nebula) [(none)]>  CREATE SPACE s1 (vid_type=int, replica_factor=1, partition_num=4) on g1
Execution succeeded (time spent 2110/2907 us)

Thu, 23 Sep 2021 17:27:40 CST

(root@nebula) [s1]> use s1;show parts
+--------------+--------+-------+-------+
| Partition ID | Leader | Peers | Losts |
+--------------+--------+-------+-------+
| 1            | ""     | ""    | ""    |
+--------------+--------+-------+-------+
| 2            | ""     | ""    | ""    |
+--------------+--------+-------+-------+
| 3            | ""     | ""    | ""    |
+--------------+--------+-------+-------+
| 4            | ""     | ""    | ""    |
+--------------+--------+-------+-------+
Got 4 rows (time spent 3046/3820 us)

Thu, 23 Sep 2021 17:28:22 CST

[darionyaphet]

#2927