fix: Ensure single newline after key with empty value in stringify output

[Neutronlul]

DISCLAIMER: I based this change off of code generated by an LLM.

This pull request attempts to fix #474.

Using test code taken from #474:

import { parseDocument } from 'yaml';
import { createPatch } from 'diff';

const yaml = `
map:
  item:

anothermap:
  anotheritem:

`;

const document = parseDocument(yaml);
const result = document.toString();

const patch = createPatch('output', yaml, result);

console.log(patch);

Before:

Index: output
===================================================================
--- output
+++ output
@@ -1,7 +1,8 @@
-
 map:
   item:

+
 anothermap:
   anotheritem:

+

After:

Index: output
===================================================================
--- output
+++ output
@@ -1,5 +1,4 @@
-
 map:
   item:

 anothermap:

As you can see, it still removes the newline at the start of the document, but as far as I can tell that's not really a problem.

I hope that the test case is sufficient and that it's in the right place, I wasn't really sure where to put it.

[eemeli]

This looks like the right fix? Had to ponder the code around this for a while, so the overall savings from having an LLM suggest a solution were pretty minimal here, they just moved the burden to me as the reviewer.

Apply the inline change, and this should be good to go?