input: Write a function to find the total number of common characters present in both given strings. The function should return the count of common characters. If either of the strings is empty, the function should return 0.
  
  assert num_common_chars("hello", "world")==2
  assert num_common_chars("abc", "def")==0
  assert num_common_chars("dog", "goddess")==3
  assert num_common_chars("", "test")==0
  assert num_common_chars("python", "typhon")==6
  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)