fix(navigation): Fix default app icon

[nickvergessen]

Before

Type: RuntimeException
Code: 0
Message: image not found: image:default-app-icon webroot: serverroot:…/server
File: …/server/lib/private/URLGenerator.php
Line: 235

After

⚙️ is shown:

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI