fix(<ContentRenderer>): prioritize default slot

[farnabaz]

🔗 Linked issue

resolves#1459

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Using the default slot in <ContentRenderer> did not affect rendered HTML when the content is a markdown file.
This PR will prioritize the empty slot and the default slot.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[atinux]

Would you mind adding a test case?