Allowing instantiating of prefab without selecting an entity in outliner (#6763)

* Allowing instantiating of prefab without selecting an entity in outliner

Signed-off-by: Mikhail Naumov <mnaumov@amazon.com>

* PR feedback

Signed-off-by: Mikhail Naumov <mnaumov@amazon.com>

Code/Framework/AzToolsFramework/AzToolsFramework/UI/Prefab/PrefabIntegrationManager.cpp

@@ -341,7 +341,8 @@ namespace AzToolsFramework
             }
 
             // Instantiate Prefab
-            if (selectedEntities.size() == 1 && !readOnlyEntityInSelection && !onlySelectedEntityIsClosedPrefabContainer)
+            if (selectedEntities.size() == 0 ||
+                selectedEntities.size() == 1 && !readOnlyEntityInSelection && !onlySelectedEntityIsClosedPrefabContainer)
             {
                 QAction* instantiateAction = menu->addAction(QObject::tr("Instantiate Prefab..."));
                 instantiateAction->setToolTip(QObject::tr("Instantiates a prefab file in the scene."));