bugfix: Fix the problem that caused highfreq's yaml to be unusable (microsoft#678)

Author: Ckend

examples/highfreq/workflow_config_High_Freq_Tree_Alpha158.yaml

@@ -59,7 +59,7 @@ task:
     record: 
         - class: "SignalRecord"
           module_path: "qlib.workflow.record_temp"
-          kwargs: 
+          kwargs: {}
         - class: "HFSignalRecord"
           module_path: "qlib.workflow.record_temp"
           kwargs: {}