Don't iterate array values if nil
Don't iterate array values if nil

I'm not quite sure what's causing this, but when we delete rows with
array values in them, we get errors on the empty rows, because we're
trying to iterate a list which is actually nil. I suspect this is a
side-effect of interpreting empty array values as [], not nil.

strategicdata/csvlint.rb,strategicdata/csvlint.rb,nickzoic/csvlint.rb