[BUG]alter tenant unset drop_after failed about cnosdb

[Benxiaohai001]

public ❯ create tenant tenant001 with drop_after = '7m';
Query took 0.004 seconds.
public ❯ select * from cluster_schema.tenants;
+-------------+--------------------------------------------------------------------------------------------------------------+
| tenant_name | tenant_options                                                                                               |
+-------------+--------------------------------------------------------------------------------------------------------------+
| cnosdb      | {"comment":"system tenant","limiter_config":null,"drop_after":null,"tenant_is_hidden":false}                 |
| tenant001   | {"comment":null,"limiter_config":null,"drop_after":{"time_num":7,"unit":"Minutes"},"tenant_is_hidden":false} |
+-------------+--------------------------------------------------------------------------------------------------------------+
Query took 0.003 seconds.
public ❯ alter tenant tenant001 unset drop_after;
Query took 0.002 seconds.
public ❯ select * from cluster_schema.tenants;
+-------------+----------------------------------------------------------------------------------------------+
| tenant_name | tenant_options                                                                               |
+-------------+----------------------------------------------------------------------------------------------+
| tenant001   | {"comment":null,"limiter_config":null,"drop_after":null,"tenant_is_hidden":false}            |
| cnosdb      | {"comment":"system tenant","limiter_config":null,"drop_after":null,"tenant_is_hidden":false} |
+-------------+----------------------------------------------------------------------------------------------+
Query took 0.002 seconds.

version:cnosdb 2.4.0, revision c4e0b74