Fixes T11593. We ask for a list of values when searching for custom "link" fields, but don't handle it correctly when actually construcitng a query.

Added this custom field:

{
  "mycompany.target-version": {
    "name": "Target Version",
    "type": "link",
    "search": true
  }
}

Set a task to "beta". Let daemons index it. Queried for:

constraints: {
  "custom.mycompany.target-version": [
    "beta"
  ]
}

Got just one result back.