Werk #13715: Fix configuration update of MRPE agent ruleset

Component	Setup
Title	Fix configuration update of MRPE agent ruleset
Date	Feb 16, 2022
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) 2.0.0p21 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

When running cmk-update-config in the process of updating Checkmk, this error could occur: "Failed to transform rule: (Ruleset: agent_config:mrpe, ...): Dictionary.transform_value() got a non-dict: None"

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