Fix nil-vs-empty-array inconsistency in buildEventChannelResourceForWebBroker policy lists

Summary

In gateway/gateway-controller/pkg/policyxds/event_channel_translator.go, the buildEventChannelResourceForWebBroker function initializes API-level and channel-level policy slices (apiOnConnectionInit, apiOnProduce, apiOnConsume, and their channel-level equivalents) as nil rather than empty slices ([]interface{}{}). When no policies are configured, these nil slices are serialized as null in the EventChannelConfig xDS resource, whereas the equivalent WebSub translator (buildEventChannelResourceForWebSub) correctly emits empty arrays for the same case.

This inconsistency in the wire format can cause unexpected behavior in the event gateway runtime when it parses policies.on_connection_init, policies.on_produce, and policies.on_consume fields.

Fix

Initialize the policy slice variables to empty slices instead of nil:

// API-level
apiOnConnectionInit := []interface{}{}
apiOnProduce := []interface{}{}
apiOnConsume := []interface{}{}

// Channel-level (inside the channels loop)
channelOnConnectionInit := []interface{}{}
channelOnProduce := []interface{}{}
channelOnConsume := []interface{}{}

References

• PR introducing the issue: https://github.com/wso2/api-platform/pull/1954 (review comment: https://github.com/wso2/api-platform/pull/1954#discussion_r3239562680)
• Related discussion: https://github.com/wso2/api-platform/pull/1944

Requested by @senthuran16.