I can confirm this, at least for PostgreSQL 9.4
{{
=# SELECT CONCAT('hello', NULL, 'there') AS salute;
salute
------------
hellothere
(1 row)
}}
The NULL elements where "collapsed" into an empty string.

However:
{{
SELECT ('hello' || NULL || 'there') AS salute;
salute
--------
<--- nothing shown here, but it's a NULL
(1 row)
}}

If any of the piped operands is NULL, the whole expression will return NULL

This is exacerbated by the fact that an HQL CONCAT function is being translated as || by many dialects, instead of respecting the explicit CONCAT call.