only warn if entities are not included

currently it warns when they are blank, too.

but it's perfectly valid to have a tweet with no entities;

``` json
{
  "created_at":"Fri Feb 07 12:01:45 +0000 2014",
  "id":431759687642787841,
  "id_str":"431759687642787841",
  "text":"Getting trains around the UK is absolutely extortionate. It never ceases to amaze and disgust me.",
  "entities":{
    "hashtags":[],
    "symbols":[],
    "urls":[],
    "user_mentions":[]
  }
}
```