Reformat empty and all-whitespace files properly

[akaihola]

Fixes #166

[akaihola]

@AckslD, would you be willing to review this pull request? Can I invite you as a collaborator on this repository?

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.29%.

Quality metrics	Before	After	Change
Complexity	2.88 ⭐	3.37 ⭐	0.49 👎
Method Length	90.00 🙂	88.10 🙂	-1.90 👍
Working memory	6.95 🙂	6.86 🙂	-0.09 👍
Quality	74.65% 🙂	74.36% 🙂	-0.29% 👎

Other metrics	Before	After	Change
Lines	327	352	25

Changed files	Quality Before	Quality After	Quality Change
src/darker/black_diff.py	75.51% ⭐	73.94% 🙂	-1.57% 👎
src/darker/tests/test_black_diff.py	73.37% 🙂	74.96% 🙂	1.59% 👍

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
src/darker/tests/test_black_diff.py	test_apply_black_excludes	4 ⭐	185 😞	11 😞	53.73% 🙂	Try splitting into smaller methods. Extract out complex expressions
src/darker/tests/test_black_diff.py	test_black_config	2 ⭐	173 😞	9 🙂	60.72% 🙂	Try splitting into smaller methods

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[AckslD]

Hi @akaihola! Sorry for my late reply! I'm afraid I'm not sure how much time I'll be able to contribute to darker. Thanks for the invite though! I'll let you know if I find more time :)