Given the root of a binary tree, return a list of lists containing the node values in level-order traversal. Each inner list should contain all values from one depth of the tree, from left to right. If the tree is empty, return an empty list.
Example 1:
Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]
Explanation: The nodes are visited one level at a time from top to bottom.
Example 2:
Input: root = [1]
Output: [[1]]
Explanation: A single-node tree has only one level.
0 <= number of nodes <= 2000-1000 <= Node.val <= 1000