Followup of https://github.com/kubernetes-sigs/gateway-api/pull/3929/files#r2222627815

Per Joel's comment, the right approach is:

• Structs that are not pointers should use omitzero
• Structs that are pointers should use omitempty

This issue intends to fix the findings on our API, assuming that Go >= 1.24 is being used, so the marshalling/unmarshalling of these structs can happen correctly and as expected by the API conventions guideline