input: Write a function named `remove_duplicate_characters` that takes a string as input and returns a new string with all duplicate characters removed. If the input string is empty, the function should return an empty string.
  assert remove_duplicate_characters("bufferoverflow")=="bufrlow"
  assert remove_duplicate_characters("abracadabra")=="abrcd"
  assert remove_duplicate_characters("Hello World")=="Helo Wrd"
  assert remove_duplicate_characters("")==""
output: def remove_duplicate_characters(s):
      seen = set()
      result = []
      for char in s:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return ''.join(result)