🐛 Bug Report: Error: Method 'toMap' cannot be called on 'BillingLimits?' because it is potentially null

### 👟 Reproduction steps

dart ./server.dart

.pub-cache/hosted/pub.dev/dart_appwrite-24.1.0/lib/src/models/project.dart:178:38: Error: Method 'toMap' cannot be called on 'BillingLimits?' because it is potentially null.
- 'BillingLimits' is from 'package:dart_appwrite/models.dart' ('.pub-cache/hosted/pub.dev/dart_appwrite-24.1.0/lib/models.dart').
Try calling using ?. instead.
"billingLimits": billingLimits.toMap(),

### 👍 Expected behavior

Compatibility with dart language standarts

### 👎 Actual Behavior

null check ignored

### 🎲 Appwrite version

Different version (specify in environment)

### 💻 Operating system

Linux

### 🧱 Your Environment

appwrite Version 1.9.0 - sorry, you won't fix appwrite version dropdown and I won't upload my env

### 👀 Have you spent some time to check if this issue has been raised before?

- [x] I checked and didn't find similar issue

### 🏢 Have you read the Code of Conduct?

- [x] I have read the [Code of Conduct](https://github.com/appwrite/appwrite/blob/HEAD/CODE_OF_CONDUCT.md)

Start with .pub-cache/hosted/pub.dev/dart_appwrite-24.1.0/lib/src/models/project.dart around line 178 and inspect the BillingLimits declaration and its generated model usage. Reproduce the Dart null-safety error with `dart ./server.dart`, then verify the project model compiles without the reported nullable-method error.

Written by the indexing model from the issue text.