fix: make parameters and results not nullable in endpoint

Author: mlopezFC

src/main/java/com/flowingcode/addons/applayout/endpoint/MenuEndpoint.java

@@ -5,6 +5,7 @@
 import com.flowingcode.addons.applayout.MenuItem;
 import com.vaadin.flow.server.auth.AnonymousAllowed;
 import com.vaadin.fusion.Endpoint;
+import com.vaadin.fusion.Nonnull;
 
 @Endpoint
 @AnonymousAllowed
@@ -16,7 +17,7 @@ public MenuEndpoint(List<MenuItemsProvider> menuItemsProviders) {
         this.menuItemsProviders = menuItemsProviders;
     }
 
-    public List<MenuItemDto> getMenuItems() {
+    public @Nonnull List<@Nonnull MenuItemDto> getMenuItems() {
         List<MenuItem> menuItems = menuItemsProviders.stream().map(MenuItemsProvider::getMenuItems).flatMap(List::stream).collect(Collectors.toList());
         return convertMenuItems(menuItems);
     }

src/main/java/com/flowingcode/addons/applayout/endpoint/MenuItemDto.java

@@ -2,15 +2,15 @@
 
 import java.util.ArrayList;
 import java.util.List;
-
 import javax.annotation.Nullable;
+import com.vaadin.fusion.Nonnull;
 
 public class MenuItemDto {
 
     private String label;
     @Nullable
     private String href;
-    private List<MenuItemDto> children = new ArrayList<>();
+    private @Nonnull List<@Nonnull MenuItemDto> children = new ArrayList<>();
 
     public MenuItemDto(String label, List<MenuItemDto> children) {
         this(label,null,children);
@@ -40,10 +40,10 @@ public String getLabel() {
     public String getHref() {
         return href;
     }
-    public void setHref(String href) {
+    public void setHref(@Nonnull String href) {
         this.href = href;
     }
-    public void setLabel(String label) {
+    public void setLabel(@Nonnull String label) {
         this.label = label;
     }