Fixes hashicorp#17727
With 3.14 to 3.15 with OP testcase:
```
[9:46](⎈ |eastus2-fxs-atlas-nonprod-fxc:ingress-controller)➜ terraform-tests/psql git:(kusto) ✗ $ tp
data.azurerm_client_config.current: Reading...
azurerm_resource_group.example: Refreshing state... [id=/subscriptions/442736d8-39d9-418c-9c0c-2cee7f5126a5/resourceGroups/azurerm-debug]
data.azurerm_client_config.current: Read complete after 0s [id=2022-07-23 07:47:07.200831 +0000 UTC]
azurerm_postgresql_server.example: Refreshing state... [id=/subscriptions/442736d8-39d9-418c-9c0c-2cee7f5126a5/resourceGroups/azurerm-debug/providers/Microsoft.DBforPostgreSQL/servers/azurerm-debug-psqlserver]
╷
│ Error: string is required
│
│ with azurerm_postgresql_active_directory_administrator.example,
│ on main.tf line 32, in resource "azurerm_postgresql_active_directory_administrator" "example":
│ 32: resource "azurerm_postgresql_active_directory_administrator" "example" {
│
╵
```
With 3.14 to my branch:
```
[9:47](⎈ |eastus2-fxs-atlas-nonprod-fxc:ingress-controller)➜ terraform-tests/psql git:(kusto) ✗ $ tfdev
[9:47](⎈ |eastus2-fxs-atlas-nonprod-fxc:ingress-controller)➜ terraform-tests/psql git:(kusto) ✗ $ tp
╷
│ Warning: Provider development overrides are in effect
│
│ The following provider development overrides are set in the CLI configuration:
│ - hashicorp/azurerm in /Users/vlazarenko/go/bin
│ - hashicorp/kubernetes in /Users/vlazarenko/go/bin
│ - favoretti/adx in /Users/vlazarenko/go/bin
│
│ The behavior may therefore not match any released version of the provider and applying changes may cause the state to become incompatible with published releases.
╵
azurerm_resource_group.example: Refreshing state... [id=/subscriptions/442736d8-39d9-418c-9c0c-2cee7f5126a5/resourceGroups/azurerm-debug]
data.azurerm_client_config.current: Reading...
data.azurerm_client_config.current: Read complete after 0s [id=2022-07-23 07:48:31.214973 +0000 UTC]
azurerm_postgresql_server.example: Refreshing state... [id=/subscriptions/442736d8-39d9-418c-9c0c-2cee7f5126a5/resourceGroups/azurerm-debug/providers/Microsoft.DBforPostgreSQL/servers/azurerm-debug-psqlserver]
azurerm_postgresql_active_directory_administrator.example: Refreshing state... [id=/subscriptions/442736d8-39d9-418c-9c0c-2cee7f5126a5/resourceGroups/azurerm-debug/providers/Microsoft.DBforPostgreSQL/servers/azurerm-debug-psqlserver]
No changes. Your infrastructure matches the configuration.
Terraform has compared your real infrastructure against your configuration and found no differences, so no changes are needed.
```