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 Community, Checkmk Pro, Checkmk Ultimate MT 1.6.0b4 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT

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