Consumer API: Data Source list - "Last Execution" related properties should be optional

If a _Data Source_ isn't associated with any _Write_ the respective booleans and numbers don't have any sense and only confuse users.

```json
{
"append": false,
"applicationId": null,
"applicationName": null,
"dataSourceName": "B.5495328944096864731.csv",
"dataSourceType": null,
"dataSourceUri": "file:/tmp/B.5495328944096864731.csv",
"durationNs": 0,
"executionEventId": null,
"executionPlanId": null,
"frameworkName": null,
"timestamp": 0
}
```