refactor: backwater curves calculator JSON definition: add allowEmpty to Z1,Z2

refs #610

src/app/calculators/courberemous/config.json

@@ -44,8 +44,14 @@
         "type": "fieldset",
         "fields": [
             "Q",
-            "Z1",
-            "Z2"
+            {
+                "id": "Z1",
+                "allowEmpty": true
+            },
+            {
+                "id": "Z2",
+                "allowEmpty": true
+            }
         ]
     },
     {