Ignore empty lines when importing registration from CSV

365Strong had an issue where the end of their .csv export contained lines like:

,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,

This was not visible in their spreadsheet editor, and OpenLifter gave an ambiguous message like "error on line 30: every lifter needs a name". We should be helpful here and just ignore lines that are entirely empty (all commas or whitespace).