azurerm_container_registry_task - fix nil pointer deref (hashicorp#31043)

[BUG] * `azurerm_container_registry_task` - prevent a panic by adding a nil check

Author: sreallymatt

internal/services/containers/container_registry_task_resource.go

@@ -1510,7 +1510,7 @@ func expandSourceRegistryCredential(input []SourceRegistryCredential) *tasks.Sou
 }
 
 func flattenSourceRegistryCredential(input *tasks.SourceRegistryCredentials) []SourceRegistryCredential {
-	if input == nil {
+	if input == nil || input.LoginMode == nil {
 		return nil
 	}