Setting a foreign key as a nullable key in model file is not working in loopback 4

### Describe the bug

I want to set up a field that is defined as a foreign key, but also supports the ability to be null. In the model file, I have this already

```
@belongsTo(() => Bill, { keyTo: 'BillID', name: 'Bill' })
BillID: Number;
```

But what I want to do is something like this

```
@property({
type: Number,
required: false,
jsonSchema: { nullable: true },
mssql: { "columnName": "BillID", "dataType": "int", "dataPrecision": null, "dataScale": null, "nullable": "YES" },
})
@belongsTo(() => Bill, { keyTo: 'BillID', name: 'Bill' })
BillID: Number;
```

but that does not work. How can I set this up?

### Logs

_No response_

### Additional information

_No response_

### Reproduction

not needed as problem is obvious

The issue identifies a model file containing the @property and @belongsTo declarations for BillID; start by tracing how LoopBack 4 represents nullable foreign keys. Done would be a confirmed supported configuration or a clarified framework change, with nullable BillID behavior verified.

Written by the indexing model from the issue text.