fix get document when missing embedded with media

in case you try to embedd a document which has media fields and that
document was deleted meanwhile there is an exception from:

```[field for field in media_fields if field in document]```

because document is `None`
Merge branch 'fix-embedded' of github.com:superdesk/eve into superdesk-fix-embedded
Fix get document when missing embbeded media
Merge branch 'superdesk-fix-embedded' into develop