TEIIDDES-2120: Fixes failing unit tests

* VdbEntry
 * Should be abstract since only ModelEntry and FileEntry are applicable
   to Vdbs

* VdbFileEntry
 * Fixes possibility of the entry name being null in circumstances where
   the FileEntryType could be null. Only cropped up in unit tests

* VdbEntryTest -> VdbFileEntryTest
 * Crazy to be testing the abstract VdbEntry being added to a Vdb when no
   such entries are ever added.
 * Use of VdbFileEntries has meant slight differences in how the tests
   are implemented.