Dropzone: fix no-file change trigger
Dropzone: fix no-file change trigger

Only set the change trigger to no-file when removing the last file.
Otherwise adding multiple files and deleting a single one results in
no-file (which is usually the initial value), resulting in the form's
submit button being disabled even though there are files ready for
upload.

DirkHoffmann/indico,mic4ael/indico,mic4ael/indico,mvidalgarcia/indico,indico/indico,pferreir/indico,DirkHoffmann/indico,indico/indico,OmeGak/indico,mvidalgarcia/indico,ThiefMaster/indico,OmeGak/indico,mic4ael/indico,indico/indico,OmeGak/indico,DirkHoffmann/indico,pferreir/indico,indico/indico,DirkHoffmann/indico,mvidalgarcia/indico,pferreir/indico,mic4ael/indico,ThiefMaster/indico,pferreir/indico,ThiefMaster/indico,mvidalgarcia/indico,OmeGak/indico,ThiefMaster/indico