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 
We aren't getting from the database, we are inserting into the database.
I'm just using the PDO default behavior. If we tell the PDO that the value is a boolean (`PDO::PARAM_BOOL`), isn't supposed that the `NULL` is converted to false? Why can't we rely on that assumption?