Werk #11493: NagVis: Fix incompatibility with PHP 7.4

Component	Other components
Title	NagVis: Fix incompatibility with PHP 7.4
Date	Sep 18, 2020
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)

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