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 
I'm not sure but I think you have to move the table creation statement to the `setUp()` method to avoid errors if the table already exists (like if you run the test suite a second time).