awful.util: table.join ignore nil arguments

Signed-off-by: koniu <gkusnierz@gmail.com>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/util.lua.in

@@ -224,6 +224,7 @@ end
 function table.join(...)
     local ret = {}
     for i = 1, arg.n do
+        if arg[i] then
             for k, v in pairs(arg[i]) do
                 if type(k) == "number" then
                     rtable.insert(ret, v)
@@ -232,6 +233,7 @@ function table.join(...)
                 end
             end
         end
+    end
     return ret
 end