azurerm_api_management: fix panic when given empty hostname_configuration

[magodo]

Fixes #11422

The panic happens with following configuration:

provider "azurerm" {
  features {}
}

resource "azurerm_resource_group" "test" {
  name = "apimgmtcrash"
  location = "eastus2"
}

resource "azurerm_api_management" "apimtest" {
  name                = "apimgmtcrash"
  location            = azurerm_resource_group.test.location
  resource_group_name = azurerm_resource_group.test.name
  publisher_name      = "Terraform"
  publisher_email     = "foo@bar.com"
  sku_name            = "Developer_1"
  identity {
    type = "SystemAssigned"
  }
  hostname_configuration {} # THIS LINE CAUSES PANIC
}

Note that this PR only avoids the panic, whilst it will cause diff in next plan:

  ~ resource "azurerm_api_management" "apimtest" {
        id                        = "/subscriptions/xxx/resourceGroups/apimgmtcrash/providers/Microsoft.ApiManagement/service/apimgmtcrash"
        name                      = "apimgmtcrash"
        tags                      = {}
        # (16 unchanged attributes hidden)

      + hostname_configuration {
        }

[tombuildsstuff]

LGTM - although the diff is expected here, since this should be omitted if these are managed via the separate resource?

[magodo]

Yes, the diff is expected even there is no azurerm_api_management_custom_domain provisioned. This is because all the nested properties inside the hostname_configuration are Optional.

[tombuildsstuff]

LGTM - confirmed this has been tested with this configuration too 👍

[ghost]

This has been released in version 2.57.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 2.57.0"
}
# ... other configuration ...

[github-actions]

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