input: Write a function to identify the longest word in a given sentence. If the sentence is empty, return an empty string.
  assert find_longest_word("The rain in Spain stays mainly in the plain.")=='mainly'
  assert find_longest_word("Hello world")=='Hello'
  assert find_longest_word("")==''
  assert find_longest_word("Python is fun")=='Python'
  assert find_longest_word("Short words")=='words'
output: def find_longest_word(sentence):
      if not sentence:
          return ''
      words = sentence.split()
      return max(words, key=len)