.github/dependabot: fix empty excludes

[ptrus]

Wanted to keep empty exclude-patterns and ignore in the config, so that one can clearly see where these should be added if needed (e.g. for dependencies that will be harder to update/will require more changes and should be updated in separate PRs).

But:

The property '#/updates/2/groups/rust/exclude-patterns' did not contain a minimum number of items 1

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[peternose]

I would add a comment to the code, similar to what is written above, just shorter.