[stats]: fix panic for empty table stats with one root chunk #12684

Job	Run time
Go tests - no race (macos-latest)	3m 45s
Go tests - no race (macos-latest)	3m 45s
Go tests - no race (ubuntu-22.04)	1m 49s
Go tests - no race (ubuntu-22.04)	1m 49s
Go tests - no race (windows-latest)	6m 43s
Go tests - no race (windows-latest)	6m 43s
Go tests (macos-latest)	11m 28s
Go tests (macos-latest)	11m 5s
Go tests (ubuntu-22.04)	22m 17s
Go tests (ubuntu-22.04)	22m 17s
Go tests (windows-latest)	20m 55s
Go tests (windows-latest)	20m 55s
	2h 13m 31s

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