input: Write a function to replace all occurrences of a character in a string with the provided replacement character. If the given string is empty, return the empty string. If the character to be replaced is not found in the string, return the original string unmodified.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Hello World!", 'o', 'Z')=="HellZ WZrld!"
  assert replace_all_occurrences("Hello World!", 'H', 'Q')=="Qello World!"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("Hello World!", 'x', 'y')=="Hello World!"
output: def replace_all_occurrences(s, char_to_replace, replacement_char):
      if not s:
          return s
      return s.replace(char_to_replace, replacement_char)