query: support tag <null /> in expected rows
query: support tag <null /> in expected rows

To express that an expected result is NULL, provide a tag <null />
to mark it. The NULL value can be generated for each database as
needed.

The <null /> is used the similar way as <value /> just it has no value.

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

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