fix(plugin-version): Correctly check void releases for a null value

js-yaml implements the YAML spec and it has no undefined type:

- nodeca/js-yaml#356 (comment)

I.e.:

```
> require('js-yaml').safeLoad('releases:\n')
{ releases: null }
```

.yarn/versions/7bfd08a0.yml

@@ -0,0 +1,2 @@
+releases:
+  "@yarnpkg/plugin-version": prerelease

packages/plugin-version/sources/versionUtils.ts

@@ -174,7 +174,7 @@ export async function updateVersionFiles(project: Project) {
     const versionContent = await xfs.readFilePromise(versionPath, `utf8`);
     const versionData = parseSyml(versionContent);
 
-    if (typeof versionData.releases === `undefined`)
+    if (versionData.releases === null)
       continue;
 
     for (const locatorStr of Object.keys(versionData.releases || {})) {