Handle missing partitions

[therve]

Both the old code and the doc worked when partition were an empty list.
This handles this case, and add tests for it and no topics.

[codecov]

Codecov Report

Merging #5035 into master will decrease coverage by 10.73%.
The diff coverage is 93.33%.

Impacted Files	Coverage Δ
...er/datadog_checks/kafka_consumer/kafka_consumer.py	70.45% <100%> (ø)
kafka_consumer/tests/test_kafka_consumer.py	94.73% <92.85%> (ø)
vertica/tests/metrics.py
...checks_dev/tests/plugin/test_environment_runner.py
oracle/tests/test_oracle.py
...r/datadog_checks/openstack_controller/__about__.py
twistlock/datadog_checks/twistlock/twistlock.py
linkerd/datadog_checks/linkerd/linkerd.py
...oller/datadog_checks/openstack_controller/utils.py
istio/tests/test_e2e.py
... and 909 more