Subscription DSL filter causes filtered events to become errors

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

### AI Policy

- [x] I agree to follow this project's AI Policy, or I agree that AI was not used while creating this issue.

### Versions

Elixir - 1.18.4
Erlang - 28.0.2
Ash - 3.5.34
AshGraphql - 1.8.2
Absinthe - 1.7.10

### Operating system

MacOS 15.6

### Current Behavior

When using the subscription DSL filter, when a filter is applied, the events that out to be excluded are still sent with a `nil` payload and an error `code: not_found`.

Example:

```gql
subscription($filter: TestResourceFilterInput) {
testResourceCreated(filter:$filter) {
created {
id
choice
}
}
}
```

```elixir
%Phoenix.Socket.Message{
topic: "__absinthe__:doc:unRcSrcuHkiBCReefuo40W42GErDK62KmQKrtqYnwyU=:1E9C1D467CBCCEB83F768CA9C9AD88068931E40CF6C347A4D1FB07F29F3762C0",
event: "subscription:data",
payload: %{result: %{data: %{"testResourceCreated" => nil},
errors: [%{code: "not_found", message: "could not be found", path: ["testResourceCreated"], fields: [], vars: [], short_message: "could not be found", locations: [%{line: 2, column: 3}]}]},
subscriptionId: "__absinthe__:doc:unRcSrcuHkiBCReefuo40W42GErDK62KmQKrtqYnwyU=:1E9C1D467CBCCEB83F768CA9C9AD88068931E40CF6C347A4D1FB07F29F3762C0"}, ref: nil, join_ref: nil
}
```

In the gql playground, this results in just a `null` event without the included error message which is particularly unhelpful.

```json
{
"data": {
"testResourceCreated": null
}
}
```

### Reproduction

[Reproduction Repo](https://github.com/justin-m-morgan/ash_gql_sub_bug)

### Expected Behavior

When a filter is applied to a subscription, excluded events should not be sent to the subscriber as error events with no data payload.

Start with the linked ash_gql_sub_bug reproduction repo and the subscription DSL filter behavior shown in the issue. Verify that a filtered-out event produces no subscription event or error, while an included event still carries its payload; use the reported GraphQL subscription and Phoenix socket message as the observable result.

Written by the indexing model from the issue text.