Simplify redability of some tests for toast_tuple_target in strings.sql

Hi all,

Some tests for toast_tuple_target introduced by c251336 check if a
toast relation is empty or not using that: 
+select 0 = pg_relation_size('pg_toast.pg_toast_'||(select oid from
pg_class where relname =
'toasttest'))/current_setting('block_size')::integer as blocks;

This is overcomplicated as there is not need to compile the relation
toast name, and reltoastrelid can be used directly, like that:
SELECT pg_relation_size(reltoastrelid) = 0 AS data_size
  FROM pg_class where relname = 'toasttest';

Any objections if I simplify those tests as per the attached?
--
Michael