Skip second WriteToc() for custom-format dumps without data.

Presently, "pg_dump --format=custom" calls WriteToc() twice.  The
second call is intended to update the data offset information,
which allegedly makes parallel pg_restore significantly faster.
However, if we aren't dumping any data, this step accomplishes
nothing and can be skipped.  This is a preparatory optimization for
follow-up commits that will move the queries for attribute
statistics to WriteToc()/_printTocEntry() to save memory.

Reviewed-by: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/Z9c1rbzZegYQTOQE%40nathan

Author: nathan-bossart

src/bin/pg_dump/pg_backup_custom.c

@@ -755,9 +755,11 @@ _CloseArchive(ArchiveHandle *AH)
 		 * If possible, re-write the TOC in order to update the data offset
 		 * information.  This is not essential, as pg_restore can cope in most
 		 * cases without it; but it can make pg_restore significantly faster
-		 * in some situations (especially parallel restore).
+		 * in some situations (especially parallel restore).  We can skip this
+		 * step if we're not dumping any data; there are no offsets to update
+		 * in that case.
 		 */
-		if (ctx->hasSeek &&
+		if (ctx->hasSeek && AH->public.dopt->dumpData &&
 			fseeko(AH->FH, tpos, SEEK_SET) == 0)
 			WriteToc(AH);
 	}