input: Write a function to sort a list of strings based on their length. The function should take a list of strings as input and return the sorted list.
  assert sort_by_length(["Hello", "World!", "Good", "Morning"]) == ["Good", "Hello", "World!", "Morning"]
  assert sort_by_length(["Python", "is", "awesome"]) == ["is", "Python", "awesome"]
  assert sort_by_length([]) == []
  assert sort_by_length(["a", "bb", "ccc", "dddd"]) == ["a", "bb", "ccc", "dddd"]
  assert sort_by_length(["short", "longer", "longest"]) == ["short", "longer", "longest"]
output: def sort_by_length(strings):
      return sorted(strings, key=len)