input: Write a function to replace a given string with another string. If the string is found in the original sentence, replace it with the new string. If the string is not found, the original sentence should be returned without changes.
  assert replace_string("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_string("Python is great", "fun", "challenging")=="Python is great"
  assert replace_string("", "old", "new")==""
  assert replace_string("I love walking on the beach", "", "something")=="I love walking on the beach"
  assert replace_string("This is a test", "is", "was")=="Thwas was a test"
output: def replace_string(sentence, old_string, new_string):
      if old_string in sentence:
          return sentence.replace(old_string, new_string)
      return sentence