add support for empty FileRecorder

csst_common/file_recorder.py

@@ -94,6 +94,9 @@ class FileRecorder(list):
 
     @property
     def summary(self):
+        if len(self) == 0:
+            return '0 alleged, 0 missing, 0 to db'
+        else:
             return f"{len(self)} alleged, " \
                    f"{len(self) - sum(self.to_table()['existence'])} missing, " \
                    f"{sum(self.to_table()['existence'])} to db"