Fix zonal_mean missing-coverage NaNs and dtype promotion

[rajeeja]

Fix #1422

• Fix non-conservative zonal_mean to prefill results with NaNs, skip empty/zero-weight slices, and promote integer/bool inputs so missing coverage is represented as
  NaN instead of zero.
• Add regression tests to ensure latitudes with no intersecting faces return NaN for both conservative and non-conservative paths, and that integer inputs are
  promoted to a floating dtype to allow NaNs.

[erogluorhan]

Thanks a lot for the quick fix! There is a link-check failure to be fixed though

[rajeeja]

Fixing link check