Error in Postgres module

The [postgres](https://registry.terraform.io/modules/sjones-sot/database-for-postgres/azurerm/0.2.0) module behaves differently in terraform 0.11.14 and 0.12. Error and input posted below. This issue is peeled off of https://github.com/Azure/terraform/issues/16

**Error**
```
Error: Missing resource instance key

on .terraform\modules\demo_db\sjones-sot-terraform-azurerm-database-for-postgres-59b1e13\postgres.tf line 10, in resource "azurerm_postgresql_server" "postgresql_server": 10: resource_group_name = "${var.resource_group_name == "" ? azurerm_resource_group.resource_group.name : var.resource_group_name}"
Because azurerm_resource_group.resource_group has "count" set, its attributes
must be accessed on specific instances.

For example, to correlate with indices of a referring resource, use:
azurerm_resource_group.resource_group[count.index]

```

**Input**
```
module "demo_db" {
source = "sjones-sot/database-for-postgres/azurerm"
resource_group_name = "${var.az_rg_name}"
location = "${var.az_region_default}"
service_name = "testdb"
db_admin_password = "${var.az_fake_password}"
sku_compute_units = 1
}
```

Reproduce the supplied module input with Terraform 0.11.14 and 0.12, then inspect .terraform/modules/demo_db/.../postgres.tf at line 10 and the resource_group definition that sets count. Done means the Postgres module no longer produces the reported missing resource instance key error under the supported Terraform version.

Written by the indexing model from the issue text.