Werk #11904: Fix TypeError in docker_container_diskstat check

Component	Checks & agents
Title	Fix TypeError in docker_container_diskstat check
Date	Feb 9, 2021
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.1.0b1 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT 2.0.0b7 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT 1.6.0p21 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT

This fixes the TypeError ('NoneType' object is not iterable) error in the docker_container_diskstat check.

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