feat: add test for null nested BelongsTo relationship

[theRealPadster]

🔗 Linked issue

1008

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

📚 Description

This adds a test for nested belongsTo relationships, and checks for nulls. Right now BaseModel.toObject() throws an error when the model has a null preloaded relationship.

TypeError: Cannot read properties of null (reading 'toObject')
    at /app/node_modules/@adonisjs/lucid/build/src/Orm/BaseModel/index.js:1507:93
    at Array.reduce (<anonymous>)
    at Proxy.toObject (/app/node_modules/@adonisjs/lucid/build/src/Orm/BaseModel/index.js:1505:56)
    at /app/node_modules/@adonisjs/lucid/build/src/Orm/BaseModel/index.js:1507:93
    at Array.reduce (<anonymous>)
    at Proxy.toObject (/app/node_modules/@adonisjs/lucid/build/src/Orm/BaseModel/index.js:1505:56)
    at /app/node_modules/@adonisjs/lucid/build/src/Orm/BaseModel/index.js:1507:93
    at Array.reduce (<anonymous>)
    at Proxy.toObject (/app/node_modules/@adonisjs/lucid/build/src/Orm/BaseModel/index.js:1505:56)
    at new AdObject (/app/app/Libraries/Algolia/classes/AdObject.ts:16:25)

Should I add the fix in this PR as well? It will resolve #1008

📝 Checklist

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

[theRealPadster]

I think this should be good to go. The test does fail. Would the best practice be to include the fix in this PR or a separate one?