input: Write a function to replace all occurrences of a specified character in a given string with the provided replacement character. If the provided character does not exist in the string, return the original string unchanged. If the string is empty, return an empty string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Hello World!", 'o', 'Y')=="HellY WYrld!"
  assert replace_all_occurrences("Hello World!", 'z', 'X')=="Hello World!"
  assert replace_all_occurrences("", 'l', 'X')==""
  assert replace_all_occurrences("No replacements needed", 'a', 'b')=="No replbcements needed"
output: def replace_all_occurrences(s, char, replacement):
      if not s:
          return ""
      if char not in s:
          return s
      return s.replace(char, replacement)