[python-package] simplify Dataset._compare_params_for_warning()

[jameslamb]

Contributes to #3756.

Private method Dataset._compare_for_warning() is used to avoid raising unnecessary warnings when creating a Dataset from another Dataset.

It's only used in one place:

LightGBM/python-package/lightgbm/basic.py

Lines 2251 to 2257 in 63a882b

	if params != reference_params:
	if not self._compare_params_for_warning(
	params=params,
	other_params=reference_params,
	ignore_keys=_ConfigAliases.get("categorical_feature")
	):
	_log_warning('Overriding the parameters from Reference Dataset.')

And that place only passes in results from Dataset.get_params().

Dataset.get_params() allways returns a dictionary (never None)...

LightGBM/python-package/lightgbm/basic.py

Line 1777 in 63a882b

def get_params(self) -> Dict[str, Any]:

... so code paths in Dataset._compare_for_warning() to handle None are unnecessary.

This PR proposes removing them.

Notes for Reviewers

I found this by checking the statement coverage of the Python package's unit tests, like this:

sh build-python.sh install

SITE_PACKAGE_DIR=$(
    python -c "import site; print(site.getsitepackages()[0])"
)
pytest \
    --cov=${SITE_PACKAGE_DIR}/lightgbm \
    --verbose \
    tests/python_package_test/

coverage html
open ./htmlcov/index.html

Found through that that these if params is None blocks are not covered by any tests, and that was my clue that they might not be necessary at all.

[jmoralez]

We should update the docstring as well

[jameslamb]

We should update the docstring as well

ah yep, sorry about that.

updated in 63c38c0

[github-actions]

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