Fix EVL failures upload error

If a cell is an empty string, it is correctly removed and there's no
attempt to convert it to an integer. Recently we've seen a sneaky
whitespace which isn't treated the same. This causes an upload failure.

- Created a test to reproduce.
- Fixed by calling string.strip() to ignore whitespace in cells.
Merge pull request #206 from alphagov/fix-evl-upload-failure

Fix EVL failures upload error