input: Write a function to find the longest word in a given sentence. Return the longest word as output. If there are multiple words with the same maximum length, return one of them. 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("Python programming is fun and easy to learn.") == "programming"
  assert find_longest_word("A quick brown fox jumps over the lazy dog.") == "quick"
  assert find_longest_word("") == ""
output: def find_longest_word(sentence):
      if not sentence:
          return ""
      words = sentence.split()
      return max(words, key=len)