input: Write a function to identify the longest word in a given sentence. The function should return the longest word as a string. If there are multiple words with the same length, return the first one encountered.
  assert find_longest_word("The rain in Spain stays mainly in the plain.")=='mainly'
  assert find_longest_word("Python is a powerful programming language.")=='programming'
  assert find_longest_word("Hello world!")=='Hello'
  assert find_longest_word("A quick brown fox jumps over the lazy dog.")=='quick'
  assert find_longest_word("")==''
output: def find_longest_word(sentence):
      words = sentence.split()
      if not words:
          return ''
      longest_word = words[0]
      for word in words:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word