[fix]: postgresql migrations apply

Sorry, I was too quick to open the previous PR without checking everything even locally.
Instead of `PASS`, it should have been `RETURN`, but also the DB type on my environment is in the `db.obj` attribute. Now my `fix` seems work.

Author: devalv

pypi_server/db/migrator/migrations/005_packagefile_basename_unique.py

@@ -6,9 +6,9 @@
 
 @migration(8)
 def add_uniquie_basename_index(migrator, db):
-    # Index already exists from previous migrations
-    if isinstance(db, PostgresqlDatabase):
-        pass
+    if isinstance(db.obj, PostgresqlDatabase):
+        # Index already exists from previous migrations
+        return
     try:
         migrate(
             migrator.add_index('packagefile', ('basename',), True)