```python
def long_word(sentence: str) -> str:
    """
    Returns the longest word in a sentence, where any run of spaces separates two words.
    If two words are equally long, the first one appears first. Returns an empty string if the sentence is empty.
    """
    if not sentence:
        return ""
    
    # Split the sentence into words using split() which splits by whitespace
    words = sentence.split()
    
    # Use max with key=len to find the longest word
    longest_word = max(words, key=len)
    
    return longest_word
```