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.

mercurial/obsutil.py

@@ -822,7 +822,7 b' def obsfateprinter(successors, markers, '
     # Date
     dates = markersdates(markers)
 
-    if verbose:
+    if dates and verbose:
         min_date = min(dates)
         max_date = max(dates)