Array field comparison behavior differs based on schema-default filling

[VarunAthreya]

Context

The diff logic has a subtle behavior difference depending on whether schema-default filling is enabled.

Behavior

When schema-default filling is enabled:

• Omitted array fields are filled with explicit nil
• This compares equal to [] that the gateway stores
• Diff reports no update needed

When schema-default filling is disabled:

• Omitted array fields remain absent from the config
• The distinction between absent and [] is preserved
• Diff correctly reports an update is needed

Example

When kong.yaml is applied first and then kong-omitted.yaml is diffed, the behavior changes based on whether skip-filling defaults is enabled.

kong.yaml

_format_version: "3.0"
plugins:
  - name: zipkin
    enabled: true
    instance_name: zipkin-empty-array
    protocols:
      - http
      - https
    config:
      http_endpoint: http://localhost:9411/api/v2/spans
      static_tags: []

kong-omitted.yaml

_format_version: "3.0"
plugins:
  - name: zipkin
    enabled: true
    instance_name: zipkin-empty-array
    protocols:
      - http
      - https
    config:
      http_endpoint: http://localhost:9411/api/v2/spans

Originally posted in #2184