provider/azurerm: `azurerm_virtual_machine` panic at UnattendConfig (#7453)

Guarding against `invalid memory address` in AdditionalUnattendConfig

```
make testacc TEST=./builtin/providers/azurerm TESTARGS='-run=TestAccAzureRMVirtualMachine_windowsUnattendedConfig'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/azurerm -v
-run=TestAccAzureRMVirtualMachine_windowsUnattendedConfig -timeout 120m
=== RUN   TestAccAzureRMVirtualMachine_windowsUnattendedConfig
--- PASS: TestAccAzureRMVirtualMachine_windowsUnattendedConfig (943.28s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/azurerm
943.299s
```

builtin/providers/azurerm/resource_arm_virtual_machine.go

@@ -730,7 +730,10 @@ func flattenAzureRmVirtualMachineOsProfileWindowsConfiguration(config *compute.W
 			c["pass"] = i.PassName
 			c["component"] = i.ComponentName
 			c["setting_name"] = i.SettingName
-			c["content"] = *i.Content
+
+			if i.Content != nil {
+				c["content"] = *i.Content
+			}
 
 			content = append(content, c)
 		}