input: Write a function to find the total number of common characters present in both strings. Each character in the result should be counted only once. The function should return the total count of unique common characters.
  
  assert num_common_chars("hello", "world") == 3
  assert num_common_chars("abc", "def") == 0
  assert num_common_chars("aaab", "abbb") == 2
  assert num_common_chars("", "abc") == 0
  assert num_common_chars("abc", "") == 0
output: def num_common_chars(s1, s2):
      set1 = set(s1)
      set2 = set(s2)
      return len(set1.intersection(set2))