fix(quickadapter): preserve zero power mean exponent in label selection

fix(quickadapter): preserve zero power mean exponent in label selection

_validate_power_mean_p returns 0.0 for p=0 (a finite float accepted by
README), and the falsy fallback silently rewrote it to 1.0, turning the
documented generalized-mean p=0 (geometric) into arithmetic. Use an
explicit None check for the fallback; map-based metrics are unchanged.

Verified in the QA image (Freqtrade 2026.8): p=0 now matches
geometric_mean selection (winner 1, identical distances), while p=None
keeps the arithmetic default.