JPA equivalent of PostgreSQL with null comparison

Question

I have a requirement something like there are two db tables A and B. Table A and table B both have column 'merit' of type integer. The requirement is to find the entries from table A those have merit that matches with the least merit number in table B.
If merit in B is NULL for all the entries, the query should result all the entries from table A.
If merit in B has valid numbers, the query should result the entries of A those match the least merit number in table B.

Sample data is like this::

      TABLE A                TABLE B
   COL1 COL2 MERIT        COL1  COL2 MERIT
    a    ab   1            c    ac    1
    b    bc   2            d    ad    3

From above data the least merit in B is 1 so, only the matching entry should result from Table A.
If merit column in B is null for all the entries ie. B has no valid number for merit, two entries from A should result.

So, I came up with the below sql query::

   select A.* from A where A.merit IS NOT DISTINCT FROM (
   select min(B.merit) from B where B.merit IS NOT NULL);

I am unable to write the JPA equivalent of this sql because of "IS NOT DISTINCT FROM".
The below queries are not working.

select a from A a where a.merit in (select min(b.merit) from B b where b.merit is not null)
select a from A a where a.merit = (select min(b.merit) from B b where b.merit is not null)

My environment is POSTGRESQL, HIBERNATE in QUARKUS.

Thanks for any suggestions.