Fix crash in commit() when all components were removed from entity

Fix crash from issue #2 and add unittest case for it. Also implements general "empty" entity support

tests/basic.d

@@ -113,6 +113,31 @@ struct EmptySystem
     int count = 0;
 }
 
+struct EntityCounterSystem
+{
+    mixin ECS.System!1;
+
+    struct EntitiesData
+    {
+        int thread_id;
+        uint length;
+        Entity[] entity;
+    }
+
+    bool onBegin()
+    {
+        count = 0;
+        return true;
+    }
+
+    void onUpdate(EntitiesData data)
+    {
+        count += data.length;
+    }
+
+    int count = 0;
+}
+
 void beforeEveryTest()
 {
     becsID!CUnregistered = ushort.max;
@@ -243,7 +268,82 @@ unittest
     gEntityManager.commit();
     entity3 = gEntityManager.getEntity(id);
     assert(!entity3.getComponent!CUnregistered);
+}
 
+@("AddEmptyEntity")
+unittest
+{
+    struct OnAddRemoveChangeCounter
+    {
+        mixin ECS.System!1;
+
+        struct EntitiesData
+        {
+            int thread_id;
+            uint length;
+            Entity[] entity;
+        }
+
+        void onAddEntity(EntitiesData data)
+        {
+            add += data.length;
+        }
+        
+        void onRemoveEntity(EntitiesData data)
+        {
+            assert(0, "It's impossible to remove entity from being updated by system which accept empty entity");
+        }
+
+        int add = 0;
+    }
+
+    gEntityManager.beginRegister();
+
+    gEntityManager.registerSystem!EntityCounterSystem(0);
+    gEntityManager.registerSystem!OnAddRemoveChangeCounter(1);
+
+    gEntityManager.endRegister();
+
+    CLong long_component = CLong(3);
+
+    Entity* entity = null;
+    EntityID entity_id = gEntityManager.addEntity(null).id;
+
+    EntityCounterSystem* system = gEntityManager.getSystem!EntityCounterSystem;
+    assert(system !is null);
+    assert(system.count == 0);
+
+    OnAddRemoveChangeCounter* add_remove_change_system = gEntityManager.getSystem!OnAddRemoveChangeCounter;
+    assert(add_remove_change_system !is null);
+    assert(add_remove_change_system.add == 0);
+
+    gEntityManager.commit();
+    assert(add_remove_change_system.add == 1);
+
+    entity = gEntityManager.getEntity(entity_id);
+    assert(!entity.hasComponent(becsID!CLong));
+    assert(entity.getComponent(becsID!CLong) is null);
+    
+
+    gEntityManager.begin();
+    gEntityManager.update();
+    assert(system.count == 1);
+    gEntityManager.end();
+
+    gEntityManager.addEntityCopy(entity_id);
+    gEntityManager.addEntityCopy(entity_id);
+    gEntityManager.addComponents(entity_id, [ComponentRef(&long_component, becsID(long_component))].staticArray);
+    gEntityManager.commit();
+    assert(add_remove_change_system.add == 3, "onAddEntity missed");
+
+    entity = gEntityManager.getEntity(entity_id);
+    assert(entity.hasComponent(becsID!CLong));
+    assert(*entity.getComponent!CLong == 3);
+
+    gEntityManager.begin();
+    gEntityManager.update();
+    assert(system.count == 3);
+    gEntityManager.end();
 }
 
 //allocate templates