4165 - Fix spurious error message when importing for oneself while having archivist privileges

Importing for yourself without entering an external author and email, with "Import for others" unticked resulted in the error message that should appear when the option is unticked and one of those fields is filled in.

https://code.google.com/p/otwarchive/issues/detail?id=4165

The code was originally checking for the presence of the external author and email fields, which are always present, rather than checking whether they were blank.