feat: allow optional relation types

[aadamcik]

🔗 Linked issue

#1098

❓ Type of change

• 🐞 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

• Resolves Nullable relations are not extracted correctly #1098 .
• This PR fixes the relation extraction when it's nullable or optional.
• It also adds support to define all relations as optional, so you have better type safety if you want to when checking whether the relation is preloaded.

📝 Checklist

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

[thetutlage]

Looks good. Thanks 👍