Director import: Using custom_values as "All custom variables" does not work

[dgoetz]

Expected Behavior

custom_values can be used by internal customers to add additional information as they need it and the monitoring administrators are not required to adjust the sync rules every time as custom_values are useable as "All custom variables".

Current Behavior

Trying to use it results in removal of all custom variables, probably because if it not being recognized as structured data like expected. Trying to use a modifier of json decode results always in custom_values being set to null, not honouring the on_failure setting.
Trying a similar thing with json data from a database and using the modifier works like expected.

Steps to Reproduce (for bugs)

Add custom attributes in vCenter, import virtual machines, try to sync the custom_values as "All custom variables".

Your Environment

• VMware vCenter®/ESXi™-Version: 7.2
• Version/GIT-Hash of this module: 1.7.1
• Icinga Web 2 version: 2.11.4
• Operating System and version: CentOS 7.9
• Webserver, PHP versions: Apache with PHP 7.3