server/rz_api_rest: return 400 on empty topo diff
server/rz_api_rest: return 400 on empty topo diff

tlevine/rhizi,tlevine/rhizi,Rhizi/rhizi,yuvadm/rhizi,Rhizi/rhizi,shlomif/rhizi,yuvadm/rhizi,shlomif/rhizi,Rhizi/rhizi,Rhizi/rhizi,tlevine/rhizi,shlomif/rhizi,cben/rhizi,ozFri/rhizi,ozFri/rhizi,cben/rhizi,tlevine/rhizi,cben/rhizi,yuvadm/rhizi,yuvadm/rhizi,ozFri/rhizi,ozFri/rhizi,shlomif/rhizi,cben/rhizi