misleading comments in pgbench

From contrib/pgbench/pgbench.c starting in revision 1.77

* Note: TPC-B requires at least 100 bytes per row, and the "filler"
* fields in these table declarations were intended to comply with that.
* But because they default to NULLs, they don't actually take any
* space. We could fix that by giving them non-null default values.
* However, that would completely break comparability of pgbench
* results with prior versions. Since pgbench has never pretended
* to be fully TPC-B compliant anyway, we stick with the historical
* behavior.

The statement about NULLs applies to the other 3 tables, but
pgbench_accounts.filler is loaded (via COPY) as empty strings, not as
NULLs. When stored into char(84), the empty string takes the full
specified storage. So on my system, rows in pgbench_accounts take up
about 130 bytes, rather than than the about 40 they would with a NULL.

The behavior probably won't be changed, but the code comment probably
should be. Sorry if this seems like picayune thing, but it led to a
bit of head scratching until I figured it out.

Cheers,

Jeff