Add test to verify null cast in boolean type

This test verify how null values are casted to boolean values in PostgresPlatform.

NULL values are wrongly casted when flag **useBooleanTrueFalseStrings** is ```true```, This issue can be related to #625 
IMO this assumption is wrong and is exactly the issue I was talking about. Why should I expect to get `false` if I have a `NULL` value in the database? There is no distinction between `NULL` and `false` anymore. Is this maybe even a PDO issue of some kind?