Don’t assume attachment has data

I believe data-less attachments can only happen with a live edit of a document object.
Merge remote-tracking branch 'origin/master' into 2391-biosample-attachment
Add same fix to antibody characterizations
Merge remote-tracking branch 'origin/master' into 2391-biosample-attachment
Merge pull request #472 from ENCODE-DCC/2391-biosample-attachment

2391 biosample empty attachment causes rendering error.