to_geotiff(compression=None) leaks a raw AttributeError instead of a clear error

[brendancol]

Describe the bug

to_geotiff(arr, path, compression=None) raises a raw AttributeError: 'NoneType' object has no attribute 'lower' from _compression_tag (xrspatial/geotiff/_encode.py).

The compression parameter is typed and documented as str (default 'zstd'), so None was never a supported value. PR #2977 added a type guard at the public boundary that rejects non-string compression, but it exempts None to keep the same guard shape as the low-level writer, so None still falls through to the downstream .lower() call.

Reproduction

import numpy as np
from xrspatial.geotiff import to_geotiff
to_geotiff(np.ones((4, 4), dtype="float32"), "out.tif", compression=None)
# AttributeError: 'NoneType' object has no attribute 'lower'

Decision needed

This needs a small design call before fixing:

• Treat None as an alias for the 'none' codec (no compression), or
• Reject None at the public boundary with the same TypeError as other invalid types.

Either one is a few lines, but the choice affects the documented contract, which is why I'm splitting it out from the type-guard fix in #2977.

Context

Surfaced during review of #2977 (issue #2975).