`group_concat()` should ignore `NULL`

[jasobrown-rs]

We do not filter out NULL values when accumulating values for mysql group_concat operations.

expected behavior

mysql> select group_concat(tag separator ':<>:') from tags where d_id = 1;
+---------------------------------------+
| group_concat(tag separator ':<>:')    |
+---------------------------------------+
| sheppard:<>:rotweiller:<>:black-brown |
+---------------------------------------+
1 row in set (0.01 sec)

actual behavior:

mysql> create cache from select group_concat(tag separator ':<>:') from tags where d_id = 1;
Query OK, 0 rows affected (0.09 sec)

mysql> select group_concat(tag separator ':<>:') from tags where d_id = 1;
+-----------------------------------------------------+
| group_concat(`noria`.`tags`.`tag` separator ':<>:') |
+-----------------------------------------------------+
| sheppard:<>:rotweiller:<>:black-brown:<>:NULL       |
+-----------------------------------------------------+
1 row in set (0.01 sec)

Note: this applies to pg string_agg() (REA-5905), as well as the existing json object agg functions.