API: Allow to pass whitespaces in MultiValue

API: Allow to pass whitespaces in MultiValue

This allows to response with an invalidreason instead silently ignore
the parameter.

Example request: api.php?format=json&action=query&titles=%20

Response before this change:
{
  "batchcomplete": ""
}

Response with this change:
{
  "batchcomplete": "",
  "query": {
    "pages": {
      "-1": {
        "title": " ",
        "invalidreason": "The requested page title is empty or contains only the name of a namespace.",
        "invalid": ""
      }
    }
  }
}

Bug: T185846
Change-Id: I6fdaf32792a0e6e37b08176f975c10607093351b