input: Write a function to find the longest word in a sentence. If there are multiple words with the same length, the function should return the first occurrence of such a word. The function should split the sentence into words using whitespace as a delimiter and compare their lengths. If the sentence is empty, the function should return an empty string.
  assert find_longest_word("The rain in Spain stays mainly in the plain.")=='mainly'
  assert find_longest_word("Python is a versatile and powerful programming language.")=='programming'
  assert find_longest_word("This is a test.")=='This'
  assert find_longest_word("")==''
output: def find_longest_word(sentence):
      if not sentence:
          return ""
      words = sentence.split()
      longest_word = ""
      for word in words:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word