Range contains element filter not using index of the element column

Hi all,
I'm wondering if there are anything to do to utilize a index when doing a
range contains element query. I have tested this with 9.6 and 12.0.

I have a table with a timestamp column that has a btree index.
I would like to do a query:
SELECT * FROM table WHERE ts <@ tsrange($1, $2, '(]');
The index is not used and a seq scan is done instead.

To use the index correctly I have to do the query like this:
SELECT * FROM table WHERE ($1 IS null OR $1 < ts) AND ($2 IS null OR ts <=
$2);
I like the <@ syntax more. Is there something I can do differently? Maybe a
different type of index instead?

Thanks,

-Lauri