NULL values incorrectly being cast to varchar for pgsql dialect

```
user=> (fmt/sqlvec (-> (dsl/update :foo) (dsl/set :foo nil)))
["update foo set foo = ?" nil]
user=> (fmt/sqlvec (-> (dsl/update :foo) (dsl/set :foo nil)) {:dialect :postgresql})
["update foo set foo = cast(? as varchar)" nil]
```

Most likely an issue with jOOQ.
