In MySQL/PostgreSQL:
SELECT concat_ws('|', 'a', '', 'b', NULL, 'c'); returns 'a||b|c'.

In SQLite (v3.44-v3.50.1):
SELECT concat_ws('|', 'a', '', 'b', NULL, 'c'); returns 'a|b|c'.

This appears to be a bug.
It correctly omits nulls.
It incorrectly omits blank strings.

The documentation states:

The concat_ws(SEP,...) function returns a string that is the concatenation of all non-null arguments beyond the first argument, using the text value of the first argument as a separator.

My bold.

Many thanks for SQLite, one of my favourite (I'm British) and most-used pieces of software!