fix(sort-keys): fix incorrect result when moving down the block located at the file start

[sumimakito]

Thank you for building this useful ESLint plugin. ❤️

Issue

When a block that will be moved down due to the sort-keys rule is located at the first line of the input, the fixed result will be incorrect.

Reproduction

https://ota-meshi.github.io/eslint-plugin-yml/playground/#eJyrVkrOT0lVslJKslIwislLtFIwjMlLtlIwVqoFAG+aB7Q=

Input

b: 2
a: 1
c: 3

Expected

a: 1
b: 2
c: 3

Actual

# An empty line (otherwise GitHub will omit the first empty line)
a: 1b: 2
c: 3

While skipping the first line can make sort-keys work correctly, for example,

---
b: 2
a: 1
c: 3

…, it's still an issue when the first line matters and it has to be moved down because of the rule.

[changeset-bot]

🦋 Changeset detected

Latest commit: 7a402b9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
eslint-plugin-yml	Patch

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[ota-meshi]

LGTM! Thanks so much!