dumpling panic when query result is empty

[D3Hunter]

Bug Report

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1. Minimal reproduce step (Required)

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x38 pc=0x1027d2684]

goroutine 44 [running]:
github.com/pingcap/tidb/dumpling/export.(*Writer).WriteTableData.func1()
	github.com/pingcap/tidb/dumpling/export/writer.go:200 +0x114
github.com/pingcap/tidb/br/pkg/utils.WithRetry({0x1036624b8, 0x14000b01860}, 0x14000a37d30, {0x12d9dad00, 0x14000a77f08})
	github.com/pingcap/tidb/br/pkg/utils/retry.go:56 +0x74
github.com/pingcap/tidb/dumpling/export.(*Writer).WriteTableData(0x14000fc36e0, {0x0, 0x0}, {0x103662528, 0x1400004fb80}, 0x0)
	github.com/pingcap/tidb/dumpling/export/writer.go:192 +0x1f4
github.com/pingcap/tidb/dumpling/export.(*Writer).handleTask(0x14000fc36e0, {0x103641c40?, 0x14000592f80?})
	github.com/pingcap/tidb/dumpling/export/writer.go:115 +0x184
github.com/pingcap/tidb/dumpling/export.(*Writer).run(0x14000fc36e0, 0x14000dfd380)
	github.com/pingcap/tidb/dumpling/export/writer.go:93 +0xc8
github.com/pingcap/tidb/dumpling/export.(*Dumper).startWriters.func4()
	github.com/pingcap/tidb/dumpling/export/dump.go:376 +0x24
golang.org/x/sync/errgroup.(*Group).Go.func1()
	golang.org/x/sync@v0.2.0/errgroup/errgroup.go:75 +0x5c
created by golang.org/x/sync/errgroup.(*Group).Go
	golang.org/x/sync@v0.2.0/errgroup/errgroup.go:72 +0x9c

introduced in #41588 where we always init SQLRowIter which causes rows closed when result is empty

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)