[bugfix] don’t crash if blank string is passed to `SomeModel.search` method

Little demo:

### URL
`http://localhost:3000/things?q[id_eq]=` (form with Ransack filter was submitted without selecting any value in it)

### Controller
```
# params == {q: {id_eq: ""}}
@q = Thing.search(params[:q])
# params == {q: {}} (because of `params.delete_if ...`)
```

### Any view that uses `params[:q]` (for example paginator)
```
= link_to "Page 2", url_for(params.merge(page: 2)) # produces http://localhost:3000/things?q=&page=2
```

The bug by itself lies in Rails but we can make a fix here.