br: restore point summary shows incorrect information if use checkpoint and restore sst

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
restore point with checkpoint

### 2. What did you expect to see? (Required)
summary shows no error
### 3. What did you see instead (Required)
total-take is incorrent and speed is NaN
### 4. What is your TiDB version? (Required)
```
[2025/01/22 15:52:48.838 +00:00] [INFO] [collector.go:77] ["restore log success summary"] [total-take=8m14.160280359s] [source-start-point=455462248614985823] [source-end-point=455479045150998556] [target-end-point=455486652481273903] [source-start="2025-01-21 09:09:54.387 +0000"] [source-end="2025-01-22 02:57:48.087 +0000"] [target-end="2025-01-22 11:01:27.749 +0000"] [total-kv-count=12147041397] [skipped-kv-count-by-checkpoint=12136187042] [total-size=677.3GB] [skipped-size-by-checkpoint=675.7GB] ["average-speed (log)"=1.371GB/s] [restore-sst-kv-count=0] [restore-sst-kv-size=0] ["restore-sst-physical-size (after compression)"=0] [restore-sst-total-take=0s] ["average-speed (sst)"=NaNB/s]
```
before this restore, there is a former restore shows restore log duration is 4h+
```
[2025/01/22 10:57:38.548 +00:00] [INFO] [restore.go:543] ["set restore kv files concurrency"] [concurrency=1025]
[2025/01/22 10:57:38.548 +00:00] [INFO] [restore.go:546] ["set restore compacted sst files concurrency per store"] [concurrency=64]
[2025/01/22 10:57:38.548 +00:00] [WARN] [restore.go:1450] ["set max-index-length to max(3072*4) to skip check index length in DDL"]
[2025/01/22 10:57:38.548 +00:00] [WARN] [restore.go:1452] ["set index-limit to max(64*8) to skip check index count in DDL"]
[2025/01/22 10:57:38.548 +00:00] [WARN] [restore.go:1454] ["set table-column-count to max(4096) to skip check column count in DDL"]
...
[2025/01/22 15:03:00.642 +00:00] [INFO] [client.go:867] ["import files done"] [batch-count=8] [batch-size=6528] [take=252.212879ms] [files="[\"v1/20250122/01/1016/455478031427043341-ccb5f737-862a-4920-a0f6-75e3ed4ed395.log, \",\"v1/20250122/01/1016/455478080605257776-6dbc26d6-bf7f-4926-9728-6221538808b3.log, \",\"v1/20250122/02/1001/455478092008521747-b96c70da-ff8b-4af9-84c0-a074fd0c1a1b.log, \",\"v1/20250122/02/1016/455478118983401543-95de8c6b-b116-4eda-bc06-a82ef8b523bb.log, \",\"v1/20250122/02/1004/455478167152099341-a90461c6-dc00-47cb-8f0a-7e7da7c03c2e.log, \",\"v1/20250122/02/1001/455478196525334605-93c587b5-2b8a-458a-8006-1d5baf536c96.log, \",\"v1/20250122/02/1004/455478227366576138-af4e9528-3652-47ee-8963-14d03deb29f6.log, \",\"v1/20250122/02/1004/455478236947939425-ed992317-5252-4eb4-ac7e-7ba5b4593c72.log, \"]"]
```

8.5.0