input: Write a function to replace all occurrences of a character in a given string with the provided replacement character. The function should take three parameters: the string to be modified, the character to be replaced, and the character to replace with. If the character to be replaced is not found in the string, the function should return the original string unchanged.
  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', 'Q')=="Hello World!"
  assert replace_all_occurrences("", 'a', 'b')==""
output: def replace_all_occurrences(s, old_char, new_char):
      if old_char not in s:
          return s
      return s.replace(old_char, new_char)