input: Write a python function to find the longest string in a list.
  assert get_longest_string(['apple', 'banana', 'cherry', 'date']) == 'banana'
  assert get_longest_string(['dog', 'elephant', 'cat', 'hippopotamus']) == 'hippopotamus'
  assert get_longest_string(['red', 'blue', 'green', 'yellow']) == 'yellow'
output: def get_longest_string(strings):
      if not strings:
          return None
      return max(strings, key=len)