`ThrowableDeserializer` does not handle `null` well for `cause`

### Describe your Issue

Currently JSON for deserializing any `Throwable` with `null` for `"cause"` property:

```
{ "message": "...",
   "cause": null
}
```

will lead to cryptic exception, due to `Throwable.initCause()` being called with `null`.
To avoid this, `ThrowableDeserializer` should handle this edge case gracefully.

