Allow users to choose no license for audio files
Allow users to choose no license for audio files

The FormHelper uses the table validation rules when it is used in the
entity context. So we need to explicitly allow the empty string for the
'audio_license' field in order to use it in the select control for
audio licenses.

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