client: Check if `border_widht` is `nil` before setting it.

Fix #3026

lib/awful/client.lua

@@ -1478,10 +1478,12 @@ for _, prop in ipairs { "border_width", "border_color", "opacity" } do
         return self["_"..prop]
     end
     client.object["set_"..prop] = function(self, value)
+        if value ~= nil then
             self._private["_user_"..prop] = true
             self["_"..prop] = value
         end
     end
+end
 
 --- Activate (focus) a client.
 --