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.

Author: oesteban

fmriprep/workflows/bold/base.py

@@ -357,7 +357,7 @@ def init_func_preproc_wf(
             fmaps.append({'suffix': 'syn'})
 
         # Short circuits: (True and True and (False or 'TooShort')) == 'TooShort'
-        run_stc = ("SliceTiming" in metadata and
+        run_stc = (metadata.get("SliceTiming") and
                    'slicetiming' not in ignore and
                    (_get_series_len(ref_file) > 4 or "TooShort"))