Correctly handle empty/null directory attribute values when using Oracle

Confluence had a bug where editing a directory would cause a data integrity violation exception on Oracle. This was because the value would get pulled in as a null (since ""==null in Oracle) and when it tried to update the value to "", it would treat it as a new row and try to do an insert again, which would fail. As a quick fix, I added code the the Embedded Crowd Hibernate 2 layer that removed all null and blank string directory attributes.

However, in a code review, Matt R pointed out that the EMBCWD code was making assumptions about the DirectoryImpl implementation (such as attributes being a mutable map) and suggested I instead move the code down to DirectoryImpl in CWD.