input: Write a function to find the total number of common characters present in both strings. Let the two strings be given as input parameters to the function. If both strings are empty, return 0.
  assert num_common_chars("hello", "world")==2
  assert num_common_chars("apple", "orange")==2
  assert num_common_chars("test", "")==0
  assert num_common_chars("", "test")==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 len(common_chars)