Stop attempts to import audio without attribution
Stop attempts to import audio without attribution

Sending the form with empty author results in an error already.
This commit just makes the form more user friendly by preventing
this to happen before sending the form.

Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2