Fix 'tiny.add' and 'tiny.remove' iterating nil values.

tiny.lua

@@ -391,12 +391,14 @@ function tiny.add(world, ...)
     local obj
     for i = 1, select("#", ...) do
         obj = select(i, ...)
+        if obj then
             if isSystem(obj) then
                 tiny_addSystem(world, obj)
             else -- Assume obj is an Entity
                 tiny_addEntity(world, obj)
             end
         end
+    end
 end
 tiny_add = tiny.add
 
@@ -427,12 +429,14 @@ function tiny.remove(world, ...)
     local obj
     for i = 1, select("#", ...) do
         obj = select(i, ...)
+        if obj then
             if isSystem(obj) then
                 tiny_removeSystem(world, obj)
             else -- Assume obj is an Entity
                tiny_removeEntity(world, obj)
             end
         end
+    end
 end
 tiny_remove = tiny.remove