return undefined when there is no documents

[mathieudutour]

fix #457

Babel would tranform [nativeDocument] = NSApplication.sharedApplication().orderedDocuments() thinking that it's a proper array but it's not, it's an NSArray so it would throw an error.

[bohemian-coding-danger]

	Warnings
⚠️	Changes were made to package.json, but not to package-lock.json - Perhaps you need to run npm run install?
⚠️	Source/dom/models/Document.js changed, but not: 📚 its docs 🧪 its tests That's OK as long as you're refactoring.

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