Support NULL arguments for properties

[AlexKirkouski]

Now DATA, GROUP and some other operators don't allow having NULL values in their parameters.

It might be really dangerous having NULL arguments in a property, because it will immediately lead to incorrect operations with sets. For example, if we have a = DATA INTEGER (A NULL), and wanna do iteration (for example FOR) over all a(b(param) objects, we will get an exception, because property will be not null for infinite number of objects. But in certain cases it is really convenient so should be implemented.

Also NULL support may be provided for comparison operators (=, >, <, ...etc,)

Probably this issue is related to #11 task.