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

tiny.lua

@@ -391,6 +391,7 @@ 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
@@ -398,6 +399,7 @@ function tiny.add(world, ...)
             end
         end
     end
+end
 tiny_add = tiny.add
 
 --- Removes an Entity to the World.
@@ -427,6 +429,7 @@ 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
@@ -434,6 +437,7 @@ function tiny.remove(world, ...)
             end
         end
     end
+end
 tiny_remove = tiny.remove
 
 -- Adds and removes Systems that have been marked from the World.