When all winddirection_10m_dominant samples were None (or the key was
absent), atan2(0, 0) produced 0.0° (due north) as a fabricated value.
Root causes:
1. [None, None, ...] is truthy → entered circular-mean branch, but
filtered sums x=y=0 → atan2(0,0)=0 → 0.0°
2. Empty list → else 0.0 branch → same fabrication
Fix: filter None from wind_d before aggregation (consistent with t_max /
t_min / uv). Only compute circular mean when at least one valid sample
exists; otherwise dominant_direction_deg and dominant_direction_label are
None. Adds TestWindDirectionAllNone covering all-None, missing key, valid
samples sanity, and mixed None+valid cases.
