Werk #7892: systemd_units: fix broken discovery rule

Component	Checks & agents
Title	systemd_units: fix broken discovery rule
Date	Jul 1, 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) 1.6.0b3 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

This fix solves the exception which occured when a systemd service discovery rule was defined

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