Fix possible IndexError

[ian28223]

What does this PR do?

Fixes a possible IndexError if subcommand returns empty

Motivation

1825-postfix-check-doesnt-work-inside-docker-container

Additional Notes

• this only addresses the IndexError resulting from the postconf binary being missing in a container.
• does not make the postconf nor postqueue available in a container which are necessary for the check to work

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #5494 into master will decrease coverage by 11.24%.
The diff coverage is 37.5%.

Impacted Files	Coverage Δ
postfix/datadog_checks/postfix/postfix.py	55% <37.5%> (-2.15%)	⬇️
postfix/tests/test_unit.py	100% <0%> (ø)	⬆️
postfix/tests/test_integration.py	100% <0%> (ø)	⬆️
vertica/tests/metrics.py
...checks_dev/tests/plugin/test_environment_runner.py
riakcs/datadog_checks/riakcs/riakcs.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
... and 995 more