Fix NPE in Preparation.

src/main/lombok/org/leumasjaffe/recipe/model/Preparation.java

@@ -17,7 +17,7 @@ import lombok.NoArgsConstructor;
 
 @Data @AllArgsConstructor @NoArgsConstructor @EqualsAndHashCode(callSuper=false)
 public class Preparation extends Observable.Instance implements RecipeComponent {
-	Duration duration;
+	Duration duration = Duration.ZERO;
 	@JsonIgnore Supplier<Collection<Ingredient>> producer = Collections::emptyList;
 	
 	@JsonIgnore