Fix todo API when used with Alerts

What does this MR do and why?

Describe in detail what your merge request does and why.

This fixes a bug where currently the TODOs API returns a 500 if an AlertManagement::Alert todo is present.

Request:

curl --location --request GET 'http://gdk.test:3000/api/v4/todos' \
--header 'PRIVATE-TOKEN: token' \

Response:

[
    {
        "id": 8,
        "project": {
            "id": 6,
            "description": "Provident doloremque blanditiis enim aut.",
            "name": "Flight",
            "name_with_namespace": "Flightjs / Flight",
            "path": "Flight",
            "path_with_namespace": "flightjs/Flight",
            "created_at": "2021-09-27T20:58:16.048Z"
        },
        "author": {
            "id": 1,
            "name": "Administrator",
            "username": "root",
            "state": "active",
            "avatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80&d=identicon",
            "web_url": "http://gdk.test:3000/root"
        },
        "action_name": "assigned",
        "target_type": "AlertManagement::Alert",
        "target": {
            "iid": 1,
            "title": "Memory usage exceeded threshold"
        },
        "target_url": "http://gdk.test:3000/flightjs/Flight/-/alert_management/1",
        "body": "Memory usage exceeded threshold",
        "state": "pending",
        "created_at": "2021-11-07T22:55:18.857Z",
        "updated_at": "2021-11-07T22:55:18.857Z"
    }
]

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Related to #345009 (closed)