Call with no subcalls is nil instead of empty array for backwards compatibility

chain/vm/fvm.go

@@ -63,14 +63,16 @@ func (t *FvmExecutionTrace) ToExecutionTrace() types.ExecutionTrace {
 		Msg:      t.Msg,
 		MsgRct:   t.MsgRct,
 		Error:    t.Error,
-		Duration:   0,
-		GasCharges: nil,
-		Subcalls:   make([]types.ExecutionTrace, len(t.Subcalls)),
+		Subcalls: nil, // Should be nil when there are no subcalls for backwards compatibility
 	}
 
+	if len(t.Subcalls) > 0 {
+		ret.Subcalls = make([]types.ExecutionTrace, len(t.Subcalls))
+
 		for i, v := range t.Subcalls {
 			ret.Subcalls[i] = v.ToExecutionTrace()
 		}
+	}
 
 	return ret
 }