Fix #343 - empty field now updates model with undefined

src/app/components/generic-input/generic-input.component.ts

@@ -244,8 +244,13 @@ export abstract class GenericInputComponent implements OnChanges {
      * met à jour le modèle d'après la saisie
      */
     public updateModelFromUI() {
-        if (this.validateUI()) {
-            this.setAndValidateModel(this, +this._uiValue); // cast UI value to Number
+        const valid = this.validateUI();
+        if (valid || this._uiValue === "") {
+            let val: number; // = undefined
+            if (valid) {
+                val = +this._uiValue; // cast UI value to Number
+            }
+            this.setAndValidateModel(this, val);
         }
     }