When signal is zero, set params to all-zeros, not nans.
When signal is zero, set params to all-zeros, not nans.

jyeatman/dipy,nilgoyyou/dipy,rfdougherty/dipy,JohnGriffiths/dipy,StongeEtienne/dipy,Messaoud-Boudjada/dipy,sinkpoint/dipy,mdesco/dipy,matthieudumont/dipy,nilgoyyou/dipy,mdesco/dipy,oesteban/dipy,oesteban/dipy,jyeatman/dipy,FrancoisRheaultUS/dipy,StongeEtienne/dipy,villalonreina/dipy,rfdougherty/dipy,sinkpoint/dipy,villalonreina/dipy,demianw/dipy,matthieudumont/dipy,beni55/dipy,beni55/dipy,FrancoisRheaultUS/dipy,Messaoud-Boudjada/dipy,JohnGriffiths/dipy,demianw/dipy