input: Write a function to find the longest common subsequence between two strings using dynamic programming. The function should return the longest common subsequence as a string. If the given strings are empty, return an empty string.
  assert longest_common_subsequence("ABCDF", "ABECD")=="ABCD"
  assert longest_common_subsequence("ABC", "AC")=="AC"
  assert longest_common_subsequence("XMJYAUZ", "MZJAWXU")=="MJAU"
  assert longest_common_subsequence("ABCDEF", "FBDAMN")=="BD"
  assert longest_common_subsequence("", "ABECD")==""
output: def longest_common_subsequence(s1, s2):
      if not s1 or not s2:
          return ""
      m, n = len(s1), len(s2)
      dp = [[""] * (n + 1) for _ in range(m + 1)]
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if s1[i - 1] == s2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + s1[i - 1]
              else:
                  dp[i][j] = max(dp[i - 1][j], dp[i][j - 1], key=len)
      return dp[m][n]