Usage of NULL in where clause fails

```
When a query has a where statement such as

where (field is not null)

It now raises an error "Expresion is not of Boolean type .!"

This used to work in earlier versions, but now fails.

Tested on D2007 Windows 7 Pro x64.
```

Original issue reported on code.google.com by `wsdah...@gmail.com` on 6 Jun 2013 at 5:20
