br: RangeTree.GetIncompleteRange return wrong result when the rangeTree is empty.

[pingyu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Run the following test case (with empty rangeTree):

func TestRangeTree(t *testing.T) {
	rangeTree := rtree.NewRangeTree()
	require.Nil(t, rangeTree.Get(newRange([]byte(""), []byte(""))))

	assertIncomplete := func(startKey, endKey []byte, ranges []rtree.Range) {
		incomplete := rangeTree.GetIncompleteRange(startKey, endKey)
		t.Logf("%#v %#v\n%#v\n%#v\n", startKey, endKey, incomplete, ranges)
		require.Equal(t, len(ranges), len(incomplete))
		for idx, rg := range incomplete {
			require.Equalf(t, ranges[idx].StartKey, rg.StartKey, "idx=%d", idx)
			require.Equalf(t, ranges[idx].EndKey, rg.EndKey, "idx=%d", idx)
		}
	}

	assertIncomplete([]byte(""), []byte(""), []rtree.Range{{StartKey: []byte(""), EndKey: []byte("")}})
}

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

Test succeed.

3. What did you see instead (Required)

Test failed:

--- FAIL: TestRangeTree (0.00s)
    /disk1/home/pingyu/workspace/tidb/br/pkg/rtree/rtree_test.go:7: []byte{} []byte{}
        []rtree.Range{}
        []rtree.Range{rtree.Range{StartKey:[]uint8{}, EndKey:[]uint8{}, Files:[]*backup.File(nil)}}
    /disk1/home/pingyu/workspace/tidb/br/pkg/rtree/rtree_test.go:8: 
        	Error Trace:	/disk1/home/pingyu/workspace/tidb/br/pkg/rtree/rtree_test.go:8
        	            				/disk1/home/pingyu/workspace/tidb/br/pkg/rtree/rtree_test.go:1
        	Error:      	Not equal: 
        	            	expected: 1
        	            	actual  : 0
        	Test:       	TestRangeTree
FAIL
FAIL	github.com/pingcap/tidb/br/pkg/rtree	0.036s
FAIL

4. What is your TiDB version? (Required)

master (96165aa)