input: Write a function to calculate the number of common substrings between two given strings. The function should return an integer indicating the count of common substrings. If either string is empty, the function should 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("abc", "def")==0
  assert num_common_substrings("", "test")==0
  assert num_common_substrings("test", "")==0
output: def num_common_substrings(s1, s2):
      if not s1 or not s2:
          return 0
      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 += k
      return count