Implement age_bands(ages) that takes a list of ages and returns a pandas Series of string labels using these right-open bands:
"0-17", "18-34", "35-54", "55+"
and band_counts(ages) returning a dict of counts for every band (including zero counts), in band order. Missing ages (None/NaN) get the label "unknown" and are counted under "unknown" only if any exist.