view: stable multi type view ::size_hint function

src/entt/entity/view.hpp

@@ -379,7 +379,7 @@ public:
      * @return Estimated number of entities iterated by the view.
      */
     [[nodiscard]] size_type size_hint() const noexcept {
-        return view->size();
+        return view ? view->size() : size_type{};
     }
 
     /**

test/entt/entity/view.cpp

@@ -630,6 +630,8 @@ TEST(MultiComponentView, InvalidView) {
 
     ASSERT_FALSE(view);
 
+    ASSERT_EQ(view.size_hint(), 0u);
+
     entt::storage<int> storage;
     view.storage(storage);