cubicweb #3440707 make the yams warnings go away [validation pending]

since yams 0.38, eschema.rdef(...) will complain if the relation type is not final and no target is asked

prioritynormal
typeenhancement
done in3.18.1
load0.100
load left0.000
closed by#0ff1de2aa55d [hooks/security] silence yams warning (closes #3440707)
patch[hooks/security] silence yams warning [applied]