def add_binary(a, b): This is to review binary operations Given two binary strings, return their sum (also a binary string). Return None if one of the input strings are empty or contains characters different than 1 or 0. Example 1: Input: a = "11", b = "1" Output: result = "100" Example 2: Input: a = "1010", b = "1011" Output: result = "10101" result = ... return result