When we merged WT-17514 (that was another hotfix) we introduced a new issue. 

The root cause is in perf_stat_collection.py values = v is unconditional, so the last iteration of the loop always overwrites values, even with an empty list.

Here's the exact scenario for read_ops and update_ops, which both have stat_files=['test.stat', 'workload.stat']:

1. Iteration 1 (test.stat): stat is found _ v = [1234.0], values = None _ condition is False _ values = [1234.0] _
2. Iteration 2 (workload.stat): file absent _ find_stat returns [] _ if v and values is False (because v is falsy) _ values = [] _ overwrites the good values with an empty list

Then add_values([]) _ self.values = [] _ average([]) _ sum([]) / len([]) _ ZeroDivisionError.