Merge pull request #5503 from peppy/fix-zero-length-hash

Fix zero-length hash models incorrectly creating a unique hash

osu.Game/Database/ArchiveModelManager.cs

@@ -253,7 +253,7 @@ private string computeHash(ArchiveReader reader)
                 using (Stream s = reader.GetStream(file))
                     s.CopyTo(hashable);
 
-            return hashable.ComputeSHA2Hash();
+            return hashable.Length > 0 ? hashable.ComputeSHA2Hash() : null;
         }
 
         /// <summary>