Werk #14832: docker_node_info: Fix inventorizing labels (which might be empty)

Component	HW/SW inventory
Title	docker_node_info: Fix inventorizing labels (which might be empty)
Date	Sep 9, 2022
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.2.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk Cloud (CCE), Checkmk MSP (CME) 2.1.0p12 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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