Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

When KIC is running in dbless mode, fields in kongPlugin.Config with null value will be omitted when sending configuration to kong:
https://github.com/Kong/kubernetes-ingress-controller/blob/1a45c0c10ef1f4c26fe63a8e3a0ab866fe7a6c01/internal/dataplane/sendconfig/inmemory.go#L52-53
The code was first introduced in PR #484.
If KIC is running in db mode, null values could be present in Kong.
This will make users unable to explicitly configure fields of plugins to null, which is meaningful in some plugins. For example, datadog plugin would use null in host, to load host from env variables. If KIC is running in dbless mode, and the config.host is set to null, the field is omitted and Kong will treat host as not specified and fill in a default value localhost.

Expected Behavior

When I explicitly specify some fields of KongPlugin.Config to null, the null value of the field should be passed to kong.

Steps To Reproduce

1. create a KongPlugin: 
apiVersion: configuration.konghq.com/v1
kind: KongPlugin
metadata:
  name: datadog
  namespace: default
  annotations:
    kubernetes.io/ingress.class: kong
plugin: datadog
config:
  host: null
  port: 8125
2. Create an ingress with annotation: konghq.com/plugins: "datadog"
3. call /plugins admin API, and the `config.host` becomes `localhost`.

Kong Ingress Controller version

v2.9.3

Kubernetes version

v1.26.3

Anything else?

https://github.com/Kong/kubernetes-ingress-controller/pull/484/files firstly introduces the cleanup of null fields in Dec 2019 and first released in KIC v0.7.0. The comment tells us Kong will return error when fields with null are present in config. Please help me to know more about the context at that time.