Checkmk Werk 13715: Fix configuration update of MRPE agent ruleset

ID: 13715 Title: Fix configuration update of MRPE agent ruleset Component: Setup Level: 1 Class: Bug fix Version: 2.1.0i1 When running <tt>cmk-update-config</tt> 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"