Fix scale and offset application when only one is available.

[sharkinsspatial]

I noted this issue recently when working with several datasets which have scales=(0.0001,) but offsets=(0.0,) resulting in the default rescale=True not applying the scale value.

To facilitate future testing of rio_reader functionality, I'd also propose incorporating some file based integration tests similar to those used in rioxarray which might cover these types of edge cases.