There is no array-concatenation function. But you can insert your NULLs
one-by-one using the '#' path notation:
~~~~~
UPDATE example
SET json=json_insert(json,'$.items.items[#]',NULL,
'$.items.items[#]',NULL,
'$.items.items[#]',NULL,
'$.items.items[#]',NULL);
~~~~~
Here is another (complete, copy/paste-able) script to illustrate:
~~~~~
CREATE TABLE t1(j JSON);
INSERT INTO t1(j) VALUES('{"a":[0,1,2,3],"b":[4,5,6,7]}');
UPDATE t1
SET j=json_insert(j,'$.a[#]',10,'$.a[#]',11,'$.a[#]',12,'$.a[#]',13);
SELECT * FROM t1;
~~~~~
The output is:
~~~~~
{"a":[0,1,2,3,10,11,12,13],"b":[4,5,6,7]}
~~~~~