Add default getEmptyValue for MultiSelect interface (#8420) (#8421)

* Add deafult getEmptyValue for MultiSelect interface (#8420)

Added an default implementation for getEmptyValue to the
MultiSelect interface.

server/src/main/java/com/vaadin/ui/AbstractMultiSelect.java

@@ -216,11 +216,6 @@ public abstract class AbstractMultiSelect<T> extends AbstractListing<T>
         updateSelection(copy, new LinkedHashSet<>(getSelectedItems()));
     }
 
-    @Override
-    public Set<T> getEmptyValue() {
-        return Collections.emptySet();
-    }
-
     /**
      * Adds a value change listener. The listener is called when the selection
      * set of this multi select is changed either by the user or

server/src/main/java/com/vaadin/ui/MultiSelect.java

@@ -136,4 +136,15 @@ public interface MultiSelect<T> extends HasValue<Set<T>>, Serializable {
      */
     public Registration addSelectionListener(
             MultiSelectionListener<T> listener);
+
+    /**
+     * MultiSelect empty value should always be an empty set by default and not
+     * {@code null}.
+     *
+     * @return An empty set, not {@code null}
+     */
+    public default Set<T> getEmptyValue() {
+        return Collections.emptySet();
+    }
+
 }