Manager: prevent error when fetching task that was never assigned

Prevent an error when fetching a task that was never assigned to a
worker.

The error:

```
WRN error fetching task worker
    error="fetching worker : worker not found: sql: no rows in result set"
```

internal/manager/api_impl/jobs_query.go

@@ -138,6 +138,7 @@ func (f *Flamenco) FetchTask(e echo.Context, taskID string) error {
 	}
 	apiTask := taskJobWorkertoAPI(taskJobWorker)
 
+	if taskJobWorker.WorkerUUID != "" {
 		// Fetch the worker. TODO: get rid of this conversion, just include the
 		// worker's UUID and let the caller fetch the worker info themselves if
 		// necessary.
@@ -147,6 +148,7 @@ func (f *Flamenco) FetchTask(e echo.Context, taskID string) error {
 			return sendAPIError(e, http.StatusInternalServerError, "error fetching task worker")
 		}
 		apiTask.Worker = workerToTaskWorker(taskWorker)
+	}
 
 	// Fetch & convert the failure list.
 	failedWorkers, err := f.persist.FetchTaskFailureList(ctx, &taskJobWorker.Task)