[http-server-csharp] Initialize collections on emit

### Clear and concise description of the problem

Right now collections are emitted like this: - (It's actually ThingLink[], but see #5945 for my collection ask)

```csharp
public ICollection ThingLinks { get; set; };
```

Without any initialization.

But that causes this warning:


I suggest initializing with `= [];`

```csharp
public ICollection ThingLinks { get; set; } = [];
```

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.