Null value directive

Special exceptions are null-value exceptions, which are fired, when operating with variables containing no value. Many operations require a selected value for a variable and cannot work properly, when no value is selected for the variable.

Statements containing null-values are ignored by default. The specific handling of null-value exceptions can, however, be controlled by the NULL_VALUE directive.

One or no option can be passed to the directive to select the current NULL_VALUE handling strategy.

accept: accepts NULL_VALUES and handles those similar to empty values with no properties.

ignore: ignores statements, which do refer to NULL_VALUES (default)

exception: throws an exception as soon as a NULL_VALUE is detected

- none -: Passing no option resets the NULL_VALUE handling strategy to the one recently set in the current expression.