`:with` and `:without` with empty arrays cause SphinxQL error

* thinking-sphinx 3.0.6 (but this was present in at least 3.0.5 and possibly before)
* sphinx 2.1.2-release

I've attached failing specs as an example.

```ruby
Book.search(:with => {:year => []}).to_a

ThinkingSphinx::SyntaxError:
     sphinxql: syntax error, unexpected ')', expecting CONST_INT or '-' near ') LIMIT 0, 20; SHOW META'
```

Not sure where the best place is to fix this. @pat, if you can advise, I'll be happy to patch this one up.