The `omitempty` JSON tag is ineffective on fields of struct type, among others.

The fields with tags changed from `omitempty` to `omitzero` can be classified into 2 categories:
- fields for which the intent was to avoid serializing the value when it was zero or empty (such as structs with the zero value for the struct);
- policy-definition fields that have the `omitempty` to match the corresponding policy fields.

The fields are changed such that
- the fields are not serialized when they have the zero (or empty) value; and
- consistency is maintained between policy fields and the corresponding policy definition fields.

---

In the context of policy definitions:

**Fields of type `struct`**:
In some cases, the fields in the policy definition and values are of type struct, such as is the case for the `policy.Policy` and `policy.Definition` structs.
In these cases, the `omitzero` JSON tag avoids marshaling empty fields, making the serialized representation more compact and less noisy, while preserving the same behavior and thus semantic when unmarshaling omitted fields.

**Fields of pointer types**:
The `omitempty` and `omitzero` have _practically_ the same effect on fields of pointer types:
- the field is omitted when it is null
- the field is included when it is not null, even if the value that it points to is "the zero value" for the non-pointer type.

Note: when the pointer type defines an `IsZero()` member function, then that field would also be omitted during marshaling. There are no defined `IsZero()` function for these pointers, so
the semantics are preserved in this case.

The `omitzero` JSON tag in the fields definition structs, such as the `ActionPolicyDefinition struct`, does not change the semantics, it simply makes the marshaled representation more compact.

**Fields of type slice**:
The behavior for `omitempty` and `omitzero` differs for slices, and maps as well.

The struct fields of slice type, such as `[]string`, are left with the `omitempty` tag to be able to tell the difference between a nil slice and a non-nil, zero-length slice. Even though, currently most code paths do not explicitly differentiate between a nil slice and an empty slice, the `omitempty` tag is left unmodified out of abundance of caution.

---

Ref:

- #4973
- #4907

- reverts 5bc467e

- Revert "chore(ci): enable modernize:omitzero linter setting (#4981)"
   reverts 06845c7
- Revert "refactor(general): omitzero JSON tag in policy structs (#4910)"
  reverts 033b4b1
- Revert "refactor(general): `omitzero` JSON tag (#4907)"
   reverts 5bc467e
- Ref: #5006