input: Write a function to find the total number of common characters present in both given strings. If either string is empty, return 0.
  assert num_common_chars("hello", "world")==2
  assert num_common_chars("abc", "def")==0
  assert num_common_chars("aaa", "a")==1
  assert num_common_chars("", "abc")==0
  assert num_common_chars("abc", "")==0
output: def num_common_chars(s1, s2):
      if not s1 or not s2:
          return 0
      common_chars = set(s1) & set(s2)
      return len(common_chars)