populate.py: crash on empty message list

[leamas]

When populate.py encounters an empty messages list around line 149 this leads to index out of bounds error in line 151 last_message = messages[-1]

A simple fix is:

index 9e93e839d23..9951af3cae2 100644
--- b/lib/populate.py
+++ a/lib/populate.py
@@ -147,6 +147,8 @@ def populate_all(
             }
 
             messages = request_all(client, request)
+            if not messages:
+                continue
 
             topic_count = len(messages)
             last_message = messages[-1]