Fix 1-dim label data

[ocefpaf]

I am hitting a

TypeError: sequence item 0: expected str instance, bytes found

with my dataset and I believe that, based on the logic in https://github.com/SciTools/iris/blob/master/lib/iris/fileformats/cf.py#L705, this should be b' ' here too,

[ocefpaf]

Thanks @bjlittle!