Bug in combo_basic: helper_get_subsequences() needs to allow length one subsequences when have_single_word = True

[bryanj1234]

In combo_basic.py, we have

def helper_get_subsequences(s: str) -> List[str]:
    """Helper function to get all subsequences of a string."""
    sequence = s.split()
    if len(sequence) <= 2:
        return []

But this means that length 1 candidate terms subsequences are not included as subsets of length 2 terms, and length 2 terms are not included as supersets of length one terms.
This is not correct when have_single_word == True.

Thanks!