[PYG-411] 🪳 Serialize direct relations without source given as string (#505)

# Description

**Reviewer**: All code inside `examples/` is generated. 

The bug is caused by direct relations without the `source` property set
to not be considered direct relations. This caused the conversions from
the `pygen` DTO (data transfer object) to instance to treat as a single
string, i.e., a regular text property instead of ensuring that it
becomes a direct relation - `dict` with space and externalId set.

## Bump

- [x] Patch
- [ ] Minor
- [ ] Skip

## Changelog
### Fixed

- Calling `.upsert()` on an instance that has a direct relations without
a source specified and with that direct relation as a string
(externalId) no longer raises a `CogniteAPIError`.
