Range contains element filter not using index of the element column

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