input: Write a function to identify the longest word in a given sentence. If there are multiple words with the same longest length, return the first one that appears. If the sentence is empty, return an empty string. The function should split the sentence into words, iterate through each word to find the longest one, and return it.
  
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("This is a test sentence.") == "sentence"
  assert find_longest_word("") == ""
  assert find_longest_word("One two three four") == "three"
  assert find_longest_word("Python programming is fun") == "programming"
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