simplify the logic, don't call_handle_subtasks() if there are no result messages
simplify the logic, don't call_handle_subtasks() if there are no result messages


Former-commit-id: 9855458d6f188c520ded92406f0aefebc6ac30c9
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine