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

[wajda]

If a Data Source isn't associated with any Write the respective booleans and numbers don't have any sense and only confuse users.

{
  "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
}