#467 [ADO.NET] DBNull.Value should be returned by CUBRIDCommand.ExecuteScalar() when the query result is null

[Jira Trac]

Description:
DBNull.Value should be returned by CUBRIDCommand.ExecuteScalar() when the query result is null. Now CUBRIDCommand.ExecuteScalar() throws an unhandled exception.
Refer to msdn: http://msdn.microsoft.com/en-us/library/system.data.common.dbcommand.executescalar.aspx

Test case is like this:
{noformat}
cmd.CommandText = select * from tablenotexist;
obj = cmd.ExecuteScalar();

Assert.AreEqual(DBNull.Value, obj);
{noformat}