vinyl: skip creating new mem on dump if the newest mem is empty

Before dumping a range, we create a new mem so that it would accommodate
statements to be inserted while we are writing the rest of mems (see
vy_task_dump_new()). Actually, we can skip this step in case the newest
mem is empty. Besides being reasonable, this micro-optimization will
come in handy for the patch moving vinyl metadata to memtx.