add dummy emtpy dataset step field to keep it a cell output by spm_jsonread

Author: Remi-Gau

model-motionDecodingMultivariate_smdl.json

@@ -8,6 +8,10 @@
     {
       "Level": "run",
       "AutoContrasts": ["trial_type.Vis_U", "trial_type.Vis_D", "trial_type.Vis_R", "trial_type.Vis_L","trial_type.Aud_U", "trial_type.Aud_D", "trial_type.Aud_R", "trial_type.Aud_L"]
+    },
+    {
+      "Level": "dataset",
+      "AutoContrasts": []
     }
   ]
 }