input: Write a function to find the longest common substring in two given strings. If the given strings do not have any common substring, return an empty string. If the input strings are empty, return an empty string as well.
  
  def longest_common_substring(str1, str2):
      match = 0
      substr = ""
      for i in range(len(str1)):
          for j in range(len(str2)):
              l = 0
              while (i + l < len(str1) and j + l < len(str2) and str1[i + l] == str2[j + l]):
                  l += 1
              if l > match:
                  substr = str1[i:i + l]
                  match = l
      return substr
  
  assert longest_common_substring("Helwallomst", "Welcome")=="l"
  assert longest_common_substring("abcdef", "xyz")==""
  assert longest_common_substring("abc", "abc")=="abc"
  assert longest_common_substring("Hello", "World")==""
  assert longest_common_substring("", "")==""
output: def longest_common_substring(str1, str2):
      match = 0
      substr = ""
      for i in range(len(str1)):
          for j in range(len(str2)):
              l = 0
              while (i + l < len(str1) and j + l < len(str2) and str1[i + l] == str2[j + l]):
                  l += 1
              if l > match:
                  substr = str1[i:i + l]
                  match = l
      return substr