author is not unknown if one was set before

[muhme]

As a user, I wish to have unknown author set if the author field is empty, even there was an author selected before.

Reproduce the error: In creating or in editing a quotation, first select an author, second clean the author field and then save. Expected is to set author unknown (as field is empty), actual behavior is previous stored author_id from hidden field is taken. This confuses users.