fix: skip undefined values in merge

[aadamcik]

🔗 Linked issue

#1111

❓ Type of change

• [x] 🐞 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)
• [x] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Resolves #1111

📝 Checklist

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