ftp: Handle NULL progress_callback cases
ftp: Handle NULL progress_callback cases

Just be precautious as we already test for this on other places.
Found by Coverity Scan analysis

xkahn/gvfs-cmis,gicmo/gvfs,halfline/gvfs,gicmo/gvfs,xkahn/gvfs-cmis,philipl/gvfs,gicmo/gvfs,philipl/gvfs,xkahn/gvfs-cmis,gicmo/gvfs,halfline/gvfs,cpatulea/gvfs,philipl/gvfs,cpatulea/gvfs,xkahn/gvfs-cmis,halfline/gvfs