Fix crash on an empty response

This check was a leftover from the resumeKey pagination, where empty responses are supposed to be impossible. With the page pagination, they are possible.

ia-cdx-search

@@ -51,7 +51,9 @@ async def wait_first_and_print(tasks):
 		return
 	task = tasks.popleft()
 	url, code, o, connection = await task
-	assert o, 'got empty response'
+	if not o:
+		print(f'Completed processing page {task._ia_cdx_page} (0 results)', file = sys.stderr)
+		return task._ia_cdx_page, connection
 	fields = o[0]
 	assert all(len(v) == len(fields) for v in o[1:]), 'got unexpected response format'
 	for row in o[1:]: