[batcher] fix random failure

https://circleci.com/gh/3scale/APIcast/18200

Author: mikz

gateway/src/apicast/policy/3scale_batcher/reports_batcher.lua

@@ -91,7 +91,7 @@ function _M:get_all(service_id)
 
     local report = keys_helper.report_from_key_batched_report(key, value)
 
-    if value and value > 0 and report.service_id == service_id then
+    if value and value > 0 and report and report.service_id == service_id then
       insert(cached_reports, report)
       self.storage:delete(key)
     end