Custom variables with `host_` or `service_` prefix have no values in tabular view

[nilmerg]

Describe the bug

Using the tabular view of a host or service list, and choosing a custom variable that is either prefixed by host_ or service_, you will get a table with said columns but their cells are all empty.

To Reproduce

1. Open the service overview
2. Choose tabular view
3. Choose columns (e.g. host name, service name, host.vars.host_prop, service.vars.service_prop
4. Look at the resulting table

Expected behavior

The cells shouldn't be empty if there are actually values available.

Your Environment

• Icinga DB Web version (System - About): main
• Icinga Web 2 version (System - About): main