NULL, empty string and values
From LimeSurvey Manual
Details on the difference between NULL, empty string and values in the database response table.
TODO
DECIMAL and DATE cannot be "empty string".
TODO: Exclusive option
TODO: Slider
Example: Slider with default value is NULL if not shown.
TODO: Relevance equation 0 with default value? Always hidden with default value?
TODO: Hidden by attribute VS hidden by relevance equation.