Allow IS NULL and IS NOT NULL comparisons for all field types.

This PR enables test for IS NULL and IS NOT NULL on all field types. Previously I would get an error like this

```
java.sql.SQLException: Field 'user_status' is of data type com.j256.ormlite.field.types.SerializableType@41479c18 which can not be compared in
```

when using QueryBuilder like this:

```java
getDao(Model.class).queryBuilder().where().isNotNull("user_status").query();
```