client/graph: find_node__by_id now returns Node/null
client/graph: find_node__by_id now returns Node/null

not undefined (undefined not appropriate for this case.

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