FIX: Do not run STC if SliceTiming metadata is set but empty
FIX: Do not run STC if SliceTiming metadata is set but empty

Currently, the BIDS-Validator would just raise a Warning when metadata contains:

```
{
  "SliceTiming": []
}

Some openneuro datasets have this.
oesteban/fmriprep,oesteban/fmriprep,poldracklab/fmriprep,poldracklab/preprocessing-workflow,oesteban/fmriprep,poldracklab/fmriprep,poldracklab/fmriprep,poldracklab/preprocessing-workflow