Werk #7495: md: Do not crash if no information about #disks, #expected disks and #working disks is available

Component	Checks & agents
Title	md: Do not crash if no information about #disks, #expected disks and #working disks is available
Date	May 20, 2019
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.6.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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