findLinked should return nothing when nothing is linked instead of every models of the linked type.

If I have a DB with several moons and a planet "earth" that has no moon linked to it , when I do: 
```javascript
 earth.get("moon").find().then(function(moons){
  console.log(moons.length);
  //it should print 0, instead it prints the total number of moons in the database
  //because moons is equal to the full list of moons
});
```
This is simply because findLinked use find underneath with an id for hasOne or a list of ids for hasMany and when id is nothing or ids is an empty list, using find is like querying everything.

**This is a real show stopper.**

So this PR is a messy trick to make sure that it will perform a request with an empty result. I know this is not the right way to do it, but this is a starting point. What do you think ?