random query generator: prefer || for postgres string concatenation

If Impala CONCAT() has an argument that evaluates to NULL, then Impala
CONCAT() returns NULL.

If PostgreSQL CONCAT() has an argument that evaluates to NULL, then
PostgreSQL will ignore it, and treat it as an empty string. Even
CONCAT(NULL) evaluates to the empty string.

PostgreSQL has a || operator that behaves like Impala CONCAT(): if a
NULL expression is on one side of ||, that || evals to NULL

For the Postgres SQL writer, we need to write logical concatenation as the || operator, not the CONCAT() function.