Allow empty values in file column

[timstallmann]

I'm trying to upload a subject set where different subjects have differing values of files, and was getting an error relating to validate_file being called and causing the whole import to fail when it didn't find a null file.

This PR modifies the subject-set upload-subjects command to:

• Skips any upload filename which is the empty string entirely
• When validate_file fails, skip that filename and continue with the rest of the upload

[timstallmann]

Hi! Just a ping to see if anyone would be able to review this?