Given the root of a binary tree, display the values of its nodes while performing a level order traversal. Return the node values for all levels in a string separated by the character :. If the tree is empty, i.e., the number of nodes is $0$, then return “None” as the output.

Constraints:

• The number of nodes in the tree is in the range $[0, 500]$.

• $-10^3 \leq$ Node.data $\leq 10^3$