Non-standard object identity test in `SparseOrbital.iter_nnz`

[ahkole]

While browsing through the implementation of SparseOrbital.iter_nnz I noticed that one of the object identity tests was the non-standard not object is None instead of object is not None, see

sisl/src/sisl/_core/sparse_geometry.py

Line 1500 in fc153f0

elif not orbitals is None:

. This is very minor and doesn't really affect the logic, but I was wondering if this should be changed nonetheless to match the standard.

[zerothi]

sure, feel free to make a PR :) thx!

[zerothi]

fixed via #996