input: Write a function to sort a list of strings by their length in ascending order. If two strings have the same length, maintain their original order in the list. The function should return the sorted list of strings.
  assert sort_by_length(["cat", "dog", "apple", "banana"])==["cat", "dog", "apple", "banana"]
  assert sort_by_length(["elephant", "bird", "ant", "hippo"])==["ant", "bird", "hippo", "elephant"]
  assert sort_by_length(["a", "bb", "ccc", "dddd"])==["a", "bb", "ccc", "dddd"]
  assert sort_by_length([""])==[""]
  assert sort_by_length([])==[]
output: def sort_by_length(strings):
      return sorted(strings, key=len)