The startup warning for missing fields is never displayed

[peteeckel]

Versions
NetBox Version: 3.6.4
NetBox DNS Version: 0.20.2

Describe the bug
On startup, NetBox DNS checks for the required IPAM custom fields to be present if feature_ipam_coupling is enabled. However, this warning never appears anywhere.

To Reproduce
Steps to reproduce the behavior:

1. Delete at least one of the custom fields starting with ipaddress_dns_
2. Set the feature_ipam_coupling setting in the plugin configuration to True
3. Restart NetBox

Expected result
A message is displayed warning about the configuration problem:

WARNING: 'feature_ipam_coupling' is enabled, but the required custom fields for IPAM-DNS coupling are missing. Please run the Django management command 'setup_coupling' to create the custom fields.```

**Actual result**
There is no warning whatsoever in any log or the system startup output, e.g. `journalctl -u netbox`, or on the command line.