input: Write a function to identify the longest word in a sentence. If the given sentence is empty then return an empty string.
  def find_longest_word(sentence):
      words = sentence.split()
      if not words:
          return ""
      longest_word = words[0]
      for word in words:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word
  
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("This is a test sentence.") == "sentence"
  assert find_longest_word("") == ""
  assert find_longest_word("OneWord") == "OneWord"
  assert find_longest_word("Short longer longest") == "longest"
output: def find_longest_word(sentence):
      words = sentence.split()
      if not words:
          return ""
      longest_word = words[0]
      for word in words:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word