ui_server: new class NullDao (generic Dao)
ui_server: new class NullDao (generic Dao)

NullDao can be used as a fast prototype of a Dao working with a
single entity. It just throws exceptions on all its methods.

Reviewed-by: Petr Stehlík <34fd672d4ba535a5cc62ffb15c811632a40953f1@stud.fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server