Werk #7972: Fix possible error when checking on command line with missing piggyback_sources directory

Component	Core & setup
Title	Fix possible error when checking on command line with missing piggyback_sources directory
Date	Jul 12, 2019
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.0.0i1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0b4 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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