Legendre azss fit range is only based on first detector

[ykyohei]

The azss fitting range is only based on first detector, which is not ideal.
This can lead to nans in fitted azss model or other issues.

sotodlib/sotodlib/tod_ops/azss.py

Line 151 in f915bfa

m = ~np.isnan(azss_stats.binned_signal[0]) # masks bins without counts

We should apply similar fix as #1596