obsfate: fix obsfate_printer with empty date list
When the list of dates is empty, `min` and `max` would raises a ValueError.
Protect against this case by checking that the date list is not empty.
I didn't add a test because I couldn't find a reproducing test case.
--- a/mercurial/obsutil.py Tue Oct 17 16:54:31 2017 +0200
+++ b/mercurial/obsutil.py Wed Oct 18 15:38:51 2017 +0200
@@ -822,7 +822,7 @@
# Date
dates = markersdates(markers)
- if verbose:
+ if dates and verbose:
min_date = min(dates)
max_date = max(dates)