input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. If the string is empty, return an empty string as well.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Welcome to Python!", 'o', 'O')=="WelcOme tO PythOn!"
  assert replace_all_occurrences("Special characters: !@#$%^&*()", ':', '-')=="Special characters- !@#$%^&*()"
  assert replace_all_occurrences("", 'a', 'b')==""
output: def replace_all_occurrences(s, char, replacement):
      if not s:
          return ""
      return s.replace(char, replacement)