Config

- Corrected typo in config.
- Added example values for taskd.trust.
Safety Valve Override

- Support 'allow.empty.filter', defaulting to 'yes', but if 'no' disallows the
  combination of a write command and an empty filter.  Ordinarily this is just
  a warning that requires confirmation (thanks to Lee Lieske).
Unit Tests

- Added filter-empty.t to test the safety valve overrides.