```python
def long_word(sentence: str) -> str:
    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
```
We've implemented the `long_word` function to find the longest word in a given sentence, following the specified guidelines. The function first checks if the input is empty and returns an empty string immediately. It then splits the sentence into words using the `split()` method and iterates through them to find the longest one. If two words of equal length are found, the first encountered wins.