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 Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 2.0.0b7 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0p21 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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

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