NullDao: remove entirely
NullDao: remove entirely

There is no need for NullDao anymore. We always prefer to use SharedPtr that
can be null while it does not lead to segfaults.

Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

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