Werk #12939: Fix crash in virtual host tree if there are no tags

Component	Checks & agents
Title	Fix crash in virtual host tree if there are no tags
Date	Oct 11, 2021
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.1.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

When creating a virtual Host tree there was an AttributeError when no TAGS are set.

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