input: Write a function to remove all consecutive duplicate characters from a given string. The function should return the modified string. If the given string is empty, return an empty string.
  
  assert remove_duplicates("AAABBCCCD")=="ABCD"
  assert remove_duplicates("AABBCC")=="ABC"
  assert remove_duplicates("")==""
  assert remove_duplicates("AAAA")=="A"
  assert remove_duplicates("ABABABA")=="ABABABA"
output: def remove_duplicates(s):
      if not s:
          return ""
      result = [s[0]]
      for char in s[1:]:
          if char != result[-1]:
              result.append(char)
      return ''.join(result)