Don't crash on packages without merges ever (very few packages)

lib/tree_builder.py

@@ -175,6 +175,10 @@ class TreeBuilder:
                 merge_before_last = last_merge
                 last_merge = factory_node
             factory_node = factory_node.parent
+
+        # a package without requests
+        if not last_merge:
+            return
         if merge_before_last:
             # we need to find the last merged_into that didn't end nowhere
             # and cut the rope there