Bug 1615408 Ensure task estimates work when none are selected r=ahal

The previous patch got backed out, and couldn't be patched itself, so here's a new one.

Differential Revision: https://phabricator.services.mozilla.com/D62844

--HG--
extra : moz-landing-system : lando

tools/tryselect/util/estimates.py

@@ -151,7 +151,11 @@ def find_longest_path(graph, tasklist, duration_data):
         return md
 
     longest_paths = [find_dependency_durations(task) for task in tasklist]
+    # Default in case there are no tasks
+    if longest_paths:
         return max(longest_paths)
+    else:
+        return 0
 
 
 def determine_quantile(quantiles_file, duration):