Geometric Series

def geometric_series(nth_term, start_term_a, common_ratio_r):
    if "" in (nth_term, start_term_a, common_ratio_r):
        return ""
    series = []
    power = 1
    multiple = common_ratio_r
    for _ in range(int(nth_term)):
        if series == []:
            series.append(start_term_a)
        else:
            power += 1
            series.append(str(float(start_term_a) * float(multiple)))
            multiple = pow(float(common_ratio_r), power)
    return series