fix: pressure loss: Strickler coefficient not empty when empty fields flag is true

refs #596

src/app/formulaire/definition/form-pressureloss.ts

 import { PressureLoss, Props, PressureLossType, Session, PressureLossLaw, CalculatorType, IObservable } from "jalhyd";
 import { FormulaireFixedVar } from "./form-fixedvar";
 import { FieldSet } from "../elements/fieldset";
+import { Prop_NullParameters } from "jalhyd";
 
 /**
  * Formulaire pour la perte de charge
@@ -27,6 +28,7 @@ export class FormulairePressureLoss extends FormulaireFixedVar {
 
         // create pressure loss law child nub
         const propsLaw: Props = new Props();
+        propsLaw.setPropValue(Prop_NullParameters, props.getPropValue(Prop_NullParameters));
         propsLaw.setPropValue("calcType", CalculatorType.PressureLossLaw);
         const law = Session.getInstance().createNub(propsLaw) as PressureLossLaw;
         const pl: PressureLoss = this.currentNub as PressureLoss;
@@ -37,7 +39,7 @@ export class FormulairePressureLoss extends FormulaireFixedVar {
         // changement de propriété du FieldSet contenant le select de choix du type de perte de charge
         if (sender instanceof FieldSet && sender.id === "fs_pressureloss_law" && data.action === "propertyChange") {
             // replace underlying pressure loss law without replacing whole Nub
-            const newPLL = Session.getInstance().createPressureLossLaw(data.value);
+            const newPLL = Session.getInstance().createPressureLossLaw(data.value, undefined, this.currentNub.getPropValue(Prop_NullParameters));
             (this._currentNub as PressureLoss).setLaw(newPLL);
             // show / hide dependent fields
             this.refreshFieldsets();