If the table is an empty, an optimization don't work when `add index`

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

If the table is empty, some optimizations don't work when add index. And if it works, that will be a bug for ingest add index.

don't worker code:

tidb/pkg/ddl/backfilling.go

Lines 676 to 678 in b1e5d61

	if startKey == nil && endKey == nil {
	return nil
	}

If works, here will return an error: ingest engine not found

tidb/pkg/ddl/index.go

Line 980 in b1e5d61

err = bc.FinishImport(indexInfo.ID, indexInfo.Unique, tbl)

2. What did you expect to see? (Required)

the optimizations can work and haven't errors.

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master