videos query event parameter doesn't match spec

DCMS v4.1.2

From `/openapi.json`:
```json
{
"description": "URL slug of event, e.g. \"dyalog-23\"",
"in": "query",
"name": "event",
"required": 0,
"schema": {
"type": "string"
}
}
```

Furthermore, it is not clear whether `?event=` should be interpreted as omitting `?event` entirely, nor how or whether that should be defined in the OpenAPI spec. Practically it makes sense to do so.

Start with the videos query handling and the `/openapi.json` definition for the `event` parameter. Determine how an empty `?event=` value should behave and update the implementation and specification so they agree; done means the parameter matches the stated API behavior.

Written by the indexing model from the issue text.