input: Write a function to determine the number of common substrings between two given strings. Return an integer indicating the count of such substrings. If either of the strings is empty, return 0.
  def num_common_substrings(s1, s2):
      count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              if s1[i] == s2[j]:
                  count += 1
      return count
  assert num_common_substrings("abcd", "bae")==2
  assert num_common_substrings("hello", "world")==1
  assert num_common_substrings("", "test")==0
  assert num_common_substrings("test", "")==0
  assert num_common_substrings("aaaa", "aa")==4
output: def num_common_substrings(s1, s2):
      count = 0
      for i in range(len(s1)):
          for j in range(len(s2)):
              k = 0
              while i + k < len(s1) and j + k < len(s2) and s1[i + k] == s2[j + k]:
                  k += 1
                  count += 1
      return count