input: Write a function to identify the longest word in a given sentence. Return the longest word as output. If there are multiple words with the same longest length, return the first one encountered.
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("He quickly realized that the treasure was buried under the old oak tree") == "quickly"
  assert find_longest_word("The small cat sleeps") == "small"
  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