feat(cpp): add the default TreeNode constructor

[j-sperling]

Problem

LeetCode's own C++ boilerplate for tree problems declares the no-argument constructor:

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

testutils/cpp/LC_IO.h declares only the latter two. So a solution that calls new TreeNode() — common in tree-building and serialize/deserialize problems — compiles on the judge and fails to compile locally:

error: no matching function for call to 'TreeNode::TreeNode()'

That is the exact case local testing exists to catch, and here it produces a false negative against code the judge accepts.

ListNode in this same header already carries the matching set of three, which makes the asymmetry easy to miss until a tree problem runs into it.

Change

One line, restoring parity with both LeetCode's boilerplate and ListNode.

Verified with the CI command (g++ -std=c++17 -O2 -o tests tests.cpp && ./tests): all 15 existing tests pass.

Filed separately from #418 so the two can be judged on their own merits — that one is a bug fix, this one touches a public struct's API.

[j178]

Thanks!