fix phone user resource

[kenchan0130]

I tried to remedy the difference on change when the computed value is null.

│ Error: Provider produced inconsistent result after apply
│
│ When applying changes to zoom_phone_user.example, provider "provider[\"registry.terraform.io/folio-sec/zoom\"]" produced an unexpected new value:
│ .site_id: was cty.StringVal(""), but now null.
│
│ This is a bug in the provider, which should be reported in the provider's own issue tracker.

However, when creating, it did not work as expected, so I have changed it acceptable to have differences when changing resources. I perceive no problem, as there is not always a difference.