Work with newer SQLAlchemy versions

Work with newer SQLAlchemy versions

Newer SQLAlchemy versions seem to track the `binary` attribute of
`BinaryMetadata`.  This means the association proxy in `DBBinary`
would first create a `BinaryMetadata` with `binary` set to `None`,
flush this to the database and then set the actual value of `binary`
we want.  However the flush fails, as `NULL` is not allowed for the
`binary_id` column.

The problem can be avoided by not setting `binary` to `None`.  This is
treated as an "undefined value" by SQLAlchemy and changing it later
means the database row never sees `NULL`.

The same applies to the `source` attribute of `SourceMetadata`.