[FTP] use an empty username if none was given
[FTP] use an empty username if none was given

fixes an assertion

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