input: Write a function to sort a list of strings by their length in ascending order. Return the sorted list. If the input list is empty, return an empty list.
  assert sort_by_length(["cat", "dog", "apple", "banana"]) == ["cat", "dog", "apple", "banana"]
  assert sort_by_length(["hello", "world", "python"]) == ["world", "hello", "python"]
  assert sort_by_length(["a", "abc", "ab"]) == ["a", "ab", "abc"]
  assert sort_by_length(["", "short", "verylongstring"]) == ["", "short", "verylongstring"]
  assert sort_by_length([]) == []
output: def sort_by_length(strings):
      return sorted(strings, key=len)