[Forcosigan]

*Note for reviewer:
This PR is based on top of PR #22. It touches SyncWorker.kt. Please review/merge that PR first.

Problem

After a scheduled task completes successfully, an additional phantom log entry immediately appears in the sync history:
Starting Sync... sync successful - There was nothing to do. Est. Speed: 0 Б/s Avg. Speed: 0 Б/s.

Cause

1. In DatabaseHandler.taskFromCursor(), cursor.getLong() was directly called for nullable columns (onFailFollowup, onSuccessFollowup). In Android SQLite, getLong() on a NULL column returns 0L.
2. When a task finished, followupTask() checked only followUpTaskID == null || followUpTaskID == -1L. Since the value was 0L, it enqueued a follow-up task with ID 0L.
3. In SyncWorker.doWork(), querying task ID 0L returned null. However, the else branch still invoked postSync() with default failureReason = NO_FAILURE and default title "Starting Sync...", falsely logging a success with 0 transferred files.

Solution

• DatabaseHandler.kt: Check cursor.isNull() before reading filterId, onFailFollowup and onSuccessFollowup, ensuring unset IDs stay null.
• SyncWorker.kt:
  • Ensure followupTask() only triggers when followUpTaskID > 0L.
  • Abort immediately in doWork() without calling postSync() when no valid task is retrieved.