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
Fix whitespace.
Add test for insertMulti with id in the last spot.

Both tests work, so there are no changes to the implementation.

Closes #4048
Fix clearing groups with FileCache and no prefix.

If you used FileCache + groups + an empty prefix cached files would not
be cleared as strpos() would fail.