pgbench - test whether a variable exists

Hello devs,

While investigating moving pgbench expressions to fe_utils so that they
can be shared with psql (\if ..., \let ?), I figure out that psql's \if
has a syntax to test whether a variable exists, which is not yet available
to pgbench.

This patch adds the same syntax to pgbench expression so that they can
represent this expression, already accepted by psql's \if.

Note that it is not really that useful for benchmarking, although it does
not harm.

--
Fabien.