upsert with adder support is not consistent with numbers and strings in LWT

postgres=# create table x (pk int PRIMARY KEY, a int, b text);
CREATE TABLE
postgres=# insert into x (pk, a, b) values(0, null, null);
INSERT 0 1
postgres=# UPDATE x SET a = a + 1 WHERE pk = 0;
UPDATE 1
postgres=# select * from x;
 pk | a | b
----+---+---
  0 |   |
(1 row)
postgres=# UPDATE x SET b = b + 'one' WHERE pk = 0;
ERROR:  operator does not exist: text + unknown
LINE 1: UPDATE x SET b = b + 'one' WHERE pk = 0;
                           ^
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.
postgres=# UPDATE x SET b = b + CAST('one' as text) WHERE pk = 0;
ERROR:  operator does not exist: text + text
LINE 1: UPDATE x SET b = b + CAST('one' as text) WHERE pk = 0;
                           ^
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.