input: Write a function to replace all occurrences of a specified character in a given string with a provided replacement character. If the string is empty, return the string as it is.
  
  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!", ' ', '-') == 'Hello-World!'
  assert replace_all_occurrences("", 'a', 'b') == ''
  assert replace_all_occurrences("Python is awesome", 'a', 'Z') == 'Python is zooesome'
output: def replace_all_occurrences(s, char_to_replace, replacement_char):
      if not s:
          return s
      return s.replace(char_to_replace, replacement_char)