Autocomplete - Do not allow the anonymous user entry to be shown

this results in having a blank entry in the autocomplete popdown which doesn't make sense.

this was discovered when confirming the other bug of adding a / to the autocomplete textfield. When adding JUST a / it will show users and a blank entry.

this patch doesn't allow the autocomplete query to perform if the string is empty causing the callback to return an empty object which is proper.