add support for empty FileRecorder

csst_common/file_recorder.py

@@ -94,9 +94,12 @@ class FileRecorder(list):
 
     @property
     def summary(self):
-        return f"{len(self)} alleged, " \
-               f"{len(self) - sum(self.to_table()['existence'])} missing, " \
-               f"{sum(self.to_table()['existence'])} to db"
+        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"
 
     def is_good(self):
         """ Check if all alleged files exist. """