Small changes in SQL scripts generating.

There is two small changes I've patched for .NET Provider.
1. First fixes problem with table with only one Id column. Inserting new values causes to generate wrong SQL query:
EXECUTE BLOCK()
RETURNS (
"Id" INT)
AS BEGIN
INSERT INTO "Users"()
VALUES ()
RETURNING "Id" INTO :"Id";
SUSPEND;
END
I've fixed generation to next:
EXECUTE BLOCK
RETURNS (
"Id" INT)
AS BEGIN
INSERT INTO "Users"
DEFAULT VALUES 
RETURNING "Id" INTO :"Id";
SUSPEND;
END
2. Second problem is absence autoincrement for identity columns. I've added creating generator and trigger to the table as described in firebird manual.

If there need tests to be added, I'll add them according to your specifications.