Silence creator and comment is required in the UI but not in the API

[gotjosh]

When creating a silence in the UI and leaving the creator and comment empty, you're met with:

However, if you try to execute a silence creation through the API (as long as you have the keys set the value would be empty).

## Create Silence
curl -X "POST" "http://localhost:9093/api/v2/silences" \
     -H 'Content-Type: application/json; charset=utf-8' \
     -d $'{
  "matchers": [
    {
      "value": "test",
      "isRegex": false,
      "isEqual": true,
      "name": "test"
    }
  ],
  "comment": "",
  "endsAt": "2022-07-11T15:37:27.659Z",
  "startsAt": "2022-07-11T13:37:27.659Z",
  "createdBy": ""
}'

This succeeds - and you get:

{
      "id":"73373e3f-a928-450a-ba75-e9254297b483",
      "status":{
         "state":"active"
      },
      "updatedAt":"2022-07-11T13:42:13.444Z",
      "comment":"",
      "createdBy":"",
      "endsAt":"2022-07-11T15:37:27.659Z",
      "matchers":[
         {
            "isEqual":true,
            "isRegex":false,
            "name":"test",
            "value":"test"
         }
      ],
      "startsAt":"2022-07-11T13:42:13.444Z"
   }

This doesn't seem right and it's kind of inconsistent with what the UI allows - should we validate at an API level that "" are not allowed?