add field index check

Co-authored-by: Matthew Frahry <mbfrahry@gmail.com>

azurerm/internal/services/kusto/kusto_cluster_resource.go

@@ -313,7 +313,7 @@ func expandKustoClusterSku(d *schema.ResourceData) (*kusto.AzureSku, error) {
 }
 
 func expandKustoClusterVNET(input []interface{}) *kusto.VirtualNetworkConfiguration {
-	if len(input) == 0 {
+	if len(input) == 0 || input[0] == nil {
 		return nil
 	}