Werk #7391: mk_mongodb: don't crash upon missing flushing info

Component	Checks & agents
Title	mk_mongodb: don't crash upon missing flushing info
Date	Apr 3, 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)

The plugin no longer crashes the server does not report info on 'backgroundFlushing', as is the case for MongoDB 4.0 onwards.

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