infrastructure improvements

shared-kernel-model/src/main/java/pl/com/bottega/factory/demand/forecasting/Demand.java

@@ -11,10 +11,6 @@ public class Demand {
         AtDayStart, Every3hours, TillDayEnd, Twice
     }
 
-    public static Demand nothingDemanded() {
-        return of(0);
-    }
-
     public static Demand of(long level) {
         return new Demand(level, Schema.TillDayEnd);
     }
@@ -22,4 +18,14 @@ public class Demand {
     public static Demand of(long level, Schema schema) {
         return new Demand(level, schema);
     }
+
+    public static Demand nothingDemanded() {
+        return of(0);
+    }
+
+    public static Demand ofNullable(Long level, Schema schema) {
+        return level == null || schema == null
+                ? nothingDemanded()
+                : of(level, schema);
+    }
 }