Add useful error on empty presets

[wooorm]

Initial checklist

• [x] I read the support docs
• [x] I read the contributing guide
• [x] I agree to follow the code of conduct
• [x] I searched issues and couldn’t find anything (or linked relevant results below)
• [x] If applicable, I’ve added docs and tests

Description of changes

Empty presets are most likely a mistake by the user. At best, they do nothing. This improves the situation for humans that make mistakes, by throwing a useful error.

Closes GH-200.

[github-actions[bot]]

Hi team! I don’t know what’s up as there’s no phase label. Please add one so I know where it’s at.

Thanks, — bb

[github-actions[bot]]

Hi team! I don’t know what’s up as there’s no phase label. Please add one so I know where it’s at.

Thanks, — bb

[github-actions[bot]]

Hi! This is accepted and can go somewhere!

Team: please review this PR and use the area/* (to describe the scope of the change), platform/* (if this is related to a specific one), and semver/* and type/* labels to annotate this.

[remcohaszing]

Since this is semver major, this would also be a good time to consider other breaking changes. The first thing that comes to mind if officially dropping support for Node.js 12.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (4a94722) 100.00% compared to head (77f3d97) 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #202 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 2 2 Lines 596 602 +6 ========================================= + Hits 596 602 +6 ``` | [Files Changed](https://app.codecov.io/gh/unifiedjs/unified/pull/202?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=unifiedjs) | Coverage Δ | | |---|---|---| | [lib/index.js](https://app.codecov.io/gh/unifiedjs/unified/pull/202?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=unifiedjs#diff-bGliL2luZGV4Lmpz) | `100.00% <100.00%> (ø)` | |

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[github-actions[bot]]

Hi! This was closed. Team: If this was merged, please describe when this is likely to be released. Otherwise, please add one of the no/* labels.