You are given a binary string s and a positive integer k. A substring of s is beautiful if the number of 1's in it is exactly k. Let len be the length of the shortest beautiful substring. Return the lexicographically smallest beautiful substring of string s with length equal to len. If s doesn't contain a beautiful substring, return an empty string. A string a is lexicographically larger than a string b (of the same length) if in the first position where a and b differ, a has a character strictly larger than the corresponding character in b.