added a test for 'is_empty' property
added a test for 'is_empty' property


Former-commit-id: 15af3a630fb6c58055f00d56ad930207bc31812a [formerly 712db407a32c7895e7474dd1bdf9947e6cda47fe] [formerly 712db407a32c7895e7474dd1bdf9947e6cda47fe [formerly 9e296f620cc7de333972fefd109ed5fbcb8087d1]]
Former-commit-id: edef7f999706ff475f5d4055f8e741f64fd570a2
Former-commit-id: b098bef5be95330c16b3e4d36d2b8ca6f2ac0683
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine