Find the Longest Word in a String

This challenge is pretty simple, you are given a string and you need to find the longest word in that string. If two words are the same length, return the first word that appears in the string. If the string is empty, return a empty string.

Examples

longest_word("Hello there") -> "Hello"
longest_word("My name is") -> "name"
longest_word("What is the longest word in this sentence") -> "sentence"