[feature request] Add support to distinguish null and {} object

Reported by https://github.com/Azure/azure-powershell/issues/17410

Autorest.powershell will treat {} as null object. And do not send {} to backend.

Beside, we are suspecting service side may report {} is invalid value if we pass {} for some other parameters (from other RPs). We are unable to predict what will be caused if we distinguish {} with null and send {} to backend. We are investigating whether we need a flag from swagger to let us know this parameter whether distinguish ‘{}’ and null.

```
"volumes": [
{
"name": "emptydir1",
"emptyDir": {}
}
]
```