input: Write a function to identify the longest word in a sentence. The function should take a sentence as input and return the longest word found in the sentence. If there are multiple words with the same maximum 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 versatile and powerful programming language.")=='programming'
  assert find_longest_word("A quick brown fox jumps over the lazy dog.")=='quick'
  assert find_longest_word("This function is designed to find the longest word.")=='designed'
  assert find_longest_word("")==''
output: def find_longest_word(sentence):
      if not sentence:
          return ''
      words = sentence.split()
      longest_word = max(words, key=len)
      return longest_word