Cannot configure `authentik_events.notificationrule` transports via blueprint

### Describe the bug

I am trying to create a notification transport rule via a blueprint. This looks something like this:

```yaml
- model: authentik_events.notificationrule
...
attrs:
...
transports:
- !If [!Condition [...],
!KeyOf global-email-notification-transport,
null]
- !If [!Condition [...],
!KeyOf global-local-notification-transport,
null]
```

The idea being that certain transports are only added if a given condition is met.

However, when any of the `!Conditions` resolves to `null`, the blueprint fails. As far as I can tell this is because the `transports` field of the `authentik_events.notificationrule` model does not have `null=True` set:

https://github.com/goauthentik/authentik/blob/6e91af6dfc0ed7626f0fcc1b72f7351782596de6/authentik/events/models.py#L598-L605

I _think_ this might be as easy to fix as adding `null=True` to the model field and running makemigrations.

Alternatively, perhaps the serialiser can be configured to ignore the null elements and only pass non-null ones to the model.

### How to reproduce

Apply a blueprint that sets `authentik_events.notificationrule.transports` field to a list of `null`s

### Expected behavior

Blueprint succeeds with only the non-null transports being added, the `null`s are ignored.

### Screenshots

_No response_

### Additional context

_No response_

### Deployment Method

Docker

### Version

2025.10.2

### Relevant log output

```shell

```