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 then return the string as it is.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Python Programming", 'o', '0')=="Pyth0n Pr0gramming"
  assert replace_all_occurrences("OpenAI is amazing", ' ', '_')=="OpenAI_is_amazing"
  assert replace_all_occurrences("RandomString", 'z', 'q')=="RandomString"
  assert replace_all_occurrences("", 'a', 'b')==""
output: def replace_all_occurrences(s, char, replacement):
      if not s:
          return s
      return s.replace(char, replacement)