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("Quick brown fox jumps over the lazy dog.") == "jumps"
  assert find_longest_word("Pneumonoultramicroscopicsilicovolcanoconiosis is a long word.") == "Pneumonoultramicroscopicsilicovolcanoconiosis"
  assert find_longest_word("") == ""
output: def find_longest_word(sentence):
      if not sentence:
          return ""
      words = sentence.split()
      return max(words, key=len)