[FEEDING SERVICE][FTP] fixed empty file detection (#1850)
[FEEDING SERVICE][FTP] fixed empty file detection (#1850)

Empty file detection was misplaced and ignoring all files (it was
testing local files before they were actually retrieved from FTP).

SDESK-5185
petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core