[meta] Improve isinstance().

* Handle nil
* Use rawequal() instead of plain ==

meta.lua

@@ -9,16 +9,20 @@ local meta = {}
 
 --- Test whether 'obj' is an instance of the given class 'cls'.
 --
--- @tparam table obj
--- @tparam table cls
+-- Note that nil is the only instance of the nil class.
+--
+-- @tparam table obj object instance to be tested
+-- @tparam table cls class argument
 -- @treturn bool
 function meta.isinstance(obj, cls)
+    if rawequal(cls, nil) then return rawequal(obj, nil) end
+
     repeat
         local m = getmetatable(obj)
         if rawequal(m, cls) then return true end
 
         obj = m
-    until obj == nil
+    until rawequal(obj, nil)
 
     return false
 end