Add the possibility to give a null parameter if the column of type resource is nullable

[TheoBiron]

Previously : the setter of a resource typed column wouldn't accept null parameter, even if the column was nullable

[coveralls]

Coverage increased (+0.004%) to 97.187% when pulling ae47a0d on TheoBiron:setter_resource_null into 1b9acd0 on thecodingmachine:master.

[moufmouf]

We need to change the unit tests to test these changes.
We should create a nullable BLOB in the test model and check the method signature.

[moufmouf]

👍 Thanks!