input: Write a function to find the number of common substrings between two given strings. Return an integer indicating the number of common substrings. If the given strings are empty then return 0.
  assert num_common_substrings("abcd", "bae")==1
  assert num_common_substrings("hello", "world")==1
  assert num_common_substrings("abc", "def")==0
  assert num_common_substrings("", "test")==0
  assert num_common_substrings("same", "same")==4
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)