input: Write a function to find the longest common substring between two given strings. If there is no common substring, return an empty string. If either of the strings is empty, return an empty string.
  assert longest_common_substring("Helwallomst", "Welcome")=="Wel"
  assert longest_common_substring("abc", "def")==""
  assert longest_common_substring("", "def")==""
  assert longest_common_substring("abc", "")==""
  assert longest_common_substring("a", "a")=="a"
output: def longest_common_substring(str1, str2):
      if not str1 or not str2:
          return ""
      m, n = len(str1), len(str2)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      max_len, end_index = 0, 0
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if str1[i - 1] == str2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + 1
                  if dp[i][j] > max_len:
                      max_len = dp[i][j]
                      end_index = i
      return str1[end_index - max_len:end_index]