fix: ENOENT assets when htmldir exists

if htmlDir already exists the previous code would not create the assets dir, this can cause ENOENT errors whenever htmlDir exists, but assets does not - for instance when outputing junit and html to the same parent folder

```
vitest --reporter junit html --outputFile.html tests/ --outputFile.junit tests/junit.xml
```

I don't understand why the previous code was checking existsSync before mkdir -p. My best guess is performance, but it's not going to have a significant effect.

packages/ui/node/reporter.ts

@@ -50,8 +50,7 @@ export default class HTMLReporter implements Reporter {
 
     const metaFile = resolve(htmlDir, 'html.meta.json')
 
-    if (!existsSync(htmlDir))
-      await fs.mkdir(resolve(htmlDir, 'assets'), { recursive: true })
+    await fs.mkdir(resolve(htmlDir, 'assets'), { recursive: true })
 
     await fs.writeFile(metaFile, report, 'utf-8')
     const ui = resolve(distDir, 'client')