Werk #4716: Fixed broken config when selecing "empty" icon as host/service icon image

Component	Core & setup
Title	Fixed broken config when selecing "empty" icon as host/service icon image
Date	May 16, 2017
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.5.0i1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.4.0b8 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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