fix binary_search_tree code

codes/csharp/chapter_tree/binary_search_tree.cs

@@ -54,9 +54,11 @@ class BinarySearchTree {
 
     /* 插入节点 */
     public void insert(int num) {
-        // 若树为空，直接提前返回
-        if (root == null)
+        // 若树为空，则初始化根节点
+        if (root == null) {
+            root = new TreeNode(num);
             return;
+        }
         TreeNode? cur = root, pre = null;
         // 循环查找，越过叶节点后跳出
         while (cur != null) {