add builder default for portfolio entity stocks

src/main/java/com/danielbohry/stocks/repository/portfolio/PortfolioEntity.java

@@ -25,6 +25,7 @@ public class PortfolioEntity {
     private Instant updatedAt;
 
     @Transient
+    @Builder.Default
     private List<PortfolioStock> stocks = new ArrayList<>();
 
     @Data