Fixed handling null values in Sqlserver->value

The value function quoted a null value as N'' instead of NULL.
As a result is produced queries like [name] IS N'' instead of [name] IS NULL which resulted in an SQL error.
Quoting test for null value and string column
Merge pull request #1620 from ovidiupruteanu/patch-1

Fixed handling null values in Sqlserver->value().

Fixes #3768