input: Write a function to find the total number of common characters present in both given strings. If both strings are empty, return 0.
  assert num_common_chars("aabbcc", "abc")==3
  assert num_common_chars("hello", "world")==2
  assert num_common_chars("aaaa", "bbbb")==0
  assert num_common_chars("", "")==0
  assert num_common_chars("python", "typing")==4
output: def num_common_chars(s1, s2):
      set1 = set(s1)
      set2 = set(s2)
      common_chars = set1 & set2
      return sum(min(s1.count(char), s2.count(char)) for char in common_chars)