BUG Fix regression introduced in #2356 (method call on non-object)

#2356 introduced a slight regression where calling `HTTP::add_cache_headers();` with no arguments caused the following error.

`Fatal error: Call to a member function getHeader() on a non-object`

See https://github.com/silverstripe/silverstripe-framework/blob/3.1.0/api/RSSFeed.php#L203 for an example of such a method call.