input: Write a function to find the total number of common characters present in both given strings. If the strings are empty then return 0.
  assert num_common_chars("hello", "world")==3
  assert num_common_chars("apple", "banana")==2
  assert num_common_chars("a", "a")==1
  assert num_common_chars("abc", "def")==0
  assert num_common_chars("", "")==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)