Don't generate doc for empty files

[fisker]

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[pkg-pr-new]

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yarn add https://pkg.pr.new/yaml-unist-parser@310.tgz

commit: 35c0227

[fisker]

@ota-meshi Looks like composer.next(cst, true) (in this pr, use composer.compose(yamlTokens, true) instead) does work without an empty doc, all the tests removed doc are empty strings.