input: Write a function to count the total number of common characters present in both given strings. If either of the strings is empty then return 0.
  assert num_common_chars("hello", "world")==2
  assert num_common_chars("aabbcc", "abc")==6
  assert num_common_chars("abcd", "efgh")==0
  assert num_common_chars("same", "same")==4
  assert num_common_chars("", "nonempty")==0
output: def num_common_chars(s1, s2):
      if not s1 or not s2:
          return 0
      common_chars = set(s1) & set(s2)
      return sum(min(s1.count(char), s2.count(char)) for char in common_chars)