input: Write a function to determine the number of common substrings between two given strings. Return an integer indicating the count of common substrings. If either of the strings is empty, return 0.
  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("a", "a")==1
  assert num_common_substrings("", "bae")==0
output: def num_common_substrings(s1, s2):
      if not s1 or not s2:
          return 0
      set1 = set()
      set2 = set()
      for i in range(len(s1)):
          for j in range(i + 1, len(s1) + 1):
              set1.add(s1[i:j])
      for i in range(len(s2)):
          for j in range(i + 1, len(s2) + 1):
              set2.add(s2[i:j])
      return len(set1 & set2)