Use NULL instead of 0 for 'arg' argument in dshash_create() calls.

A couple of dshash_create() callers provide 0 for the 'void *arg'
argument, which might give readers the incorrect impression that
this is some sort of "flags" parameter.

Reviewed-by: Andy Fan
Discussion: https://postgr.es/m/20240119215941.GA1322079%40nathanxps13

src/backend/replication/logical/launcher.c

@@ -1013,7 +1013,7 @@ logicalrep_launcher_attach_dshmem(void)
 		last_start_times_dsa = dsa_create(LWTRANCHE_LAUNCHER_DSA);
 		dsa_pin(last_start_times_dsa);
 		dsa_pin_mapping(last_start_times_dsa);
-		last_start_times = dshash_create(last_start_times_dsa, &dsh_params, 0);
+		last_start_times = dshash_create(last_start_times_dsa, &dsh_params, NULL);
 
 		/* Store handles in shared memory for other backends to use. */
 		LogicalRepCtx->last_start_dsa = dsa_get_handle(last_start_times_dsa);

src/backend/utils/activity/pgstat_shmem.c

@@ -180,7 +180,7 @@ StatsShmemInit(void)
 		 * With the limit in place, create the dshash table. XXX: It'd be nice
 		 * if there were dshash_create_in_place().
 		 */
-		dsh = dshash_create(dsa, &dsh_params, 0);
+		dsh = dshash_create(dsa, &dsh_params, NULL);
 		ctl->hash_handle = dshash_get_hash_table_handle(dsh);
 
 		/* lift limit set above */