Handle BOM and other issues found in forum issue

[caro401]

Forum post: https://forums.tropy.org/t/import-metadata-from-csv/2784/5

Things that were broken

• plugin didn't handle files with BOM at all. This is just an option in the CSV parsing library I'd forgotten to add
• plugin broke when there was a missing value in a TROPY/#path column (corresponding to a missing photo filename) because it couldn't get the file extension of undefined. This is fixed by checking the path is defined when trying to construct a photo
• plugin broke when some columns were empty, fixed by returning an empty object in these cases, rather than undefined (Plugin fails when first column of row contains no data #23)

I'm sure there's better ways to handle at least the last 2 issues, but this was the best I could think of now.