Create a method named countChildSumToEven that accepts a single BinaryTree<Int>? and counts the number of nodes in the tree where the values of the two children sum to an even number. You should not count nodes that don't have at least one child. Keep in mind that not every node has a right or left child, so you'll need to check for null carefully. However, you can assume that all the values in the tree are Ints.

For reference, cs125.trees.BinaryTree is defined like this: