Return a TermCounts object if the user provides an annotation file, otherwise None.

def get_tcntobj(go2obj, **kws):
    """Return a TermCounts object if the user provides an annotation file, otherwise None."""
    # kws: gaf gene2go
    annots = read_annotations(**kws)
    if annots:
        return TermCounts(go2obj, annots)