input: Write a function to identify the longest word in a given sentence. If there are multiple words with the same length, return the first one encountered. 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("A quick brown fox jumps over the lazy dog.")== "quick"
  assert find_longest_word("Python programming is fun")== "programming"
  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