Fix an uncaught python exception in timing

Previously, when either the 'before' or 'after' had no files, we'd get
an uncaught exception:
```
Traceback (most recent call last):
  File "/home/jgross/Documents/repos/coq/tools/make-both-time-files.py", line 16, in <module>
    table = make_diff_table_string(left_dict, right_dict, sort_by=args.sort_by, include_mem=args.include_mem, sort_by_mem=args.sort_by_mem)
  File "/home/jgross/Documents/repos/coq/tools/TimeFileMaker.py", line 391, in make_diff_table_string
    right_peak = max(v.get(MEM_KEY, 0) for v in right_dict.values())
ValueError: max() arg is an empty sequence
```

Fixes #12387

0 files changed, 0 insertions, 0 deletions