XAML Source Generation doesn't check for null references

### Description

This means that bindings where intermediate properties are null will cause NullReferenceExceptions instead of being silently tolerated as runtime and XamlC bindings did in the past.

This makes it unnecessarily incompatible with many existing codebases. Switching to XAML Source Generation is too much overhead because every binding in every page needs to be verified at runtime.

Emitted code like this:

```c#
getter: source => (source.A.B.C.Text, true),
```

could easily be A?.B?.C instead, without a significant performance loss.

At the very least, an option could be provided to generate code with null checks. Or syntax like `{Binding A?.B?.C?.Text}` could be introduced.

### Steps to Reproduce

1. Run the repro app. It will crash with a NullReferenceException.

### Link to public reproduction project repository

https://github.com/mfeingol/repros/tree/main/MauiSourceGenRepro

### Version with bug

10.0.80

### Is this a regression from previous behavior?

No, this is something new

### Last version that worked well

Unknown/Other

### Affected platforms

Android

### Affected platform versions

Android 16

### Did you find any workaround?

Remain on XamlC

### Relevant log output

```shell

```