gluon-core: fix handling of 'disabled' site.conf attributes for mesh interfaces

Because is_disabled() was always returning true or false, the
first_non_nil() would never actually check the default setting from
site.conf. This was broken since v2017.1.

Fixes: 6cf03bab ("treewide: replace normal uses of luci.model.uci with simple-uci to reduce LuCI dependencies")

package/gluon-core/luasrc/lib/gluon/upgrade/200-wireless

@@ -85,7 +85,7 @@ local function is_disabled(name)
 	if uci:get('wireless', name) then
 		return uci:get_bool('wireless', name, 'disabled')
 	else
-		return false
+		return nil
 	end
 end