Reintroduce NoNullGuessingAssertionModel.
Make NoNullGuessingAssertionModel default for experiment.
Revert "Make NoNullGuessingAssertionModel default for experiment."

This reverts commit aeadf25e43feafd8beef7c36139179a80ab096c8.
Revert "Reintroduce NoNullGuessingAssertionModel."

This reverts commit 1a23f685dfd6943481da914ad33263964bcdff72.
Make NoNullGuessingObjectChooser default for experiment.
Define test case for experiment.