input: Write a function to count the number of common substrings between two given strings. The function should return an integer indicating the count of such substrings. If no common substrings exist, 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("python", "java") == 0
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