[import] SHOW IMPORT JOB returns a nil pointer error when a finished job has NULL summary

## Bug Report

### 1. Minimal reproduce step (Required)

1. Complete any successful `IMPORT INTO` job and note its job ID. The job must be in the `finished` state.
2. As a privileged user, make the persisted job metadata inconsistent by clearing its optional summary:

```sql
UPDATE mysql.tidb_import_jobs
SET summary = NULL
WHERE id = AND status = 'finished';
```

3. Query the job:

```sql
SHOW IMPORT JOB ;
```

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

`SHOW IMPORT JOB` should return the job record without failing. If the persisted summary is missing, the imported-row/conflicted-row fields should be represented as `NULL`, zero, or another documented unavailable value.

### 3. What did you see instead (Required)

The statement fails with:

```text
ERROR 1105 (HY000):
runtime error: invalid memory address or nil pointer dereference
```

The failure is in the successful-job display path, which dereferences `Summary` without checking whether it is `NULL`.

This is a robustness issue for inconsistent Import job metadata. Normal Import completion currently persists a non-`NULL` summary, so the reproduction intentionally creates the inconsistent `finished` + `summary IS NULL` state. The behavior was directly reproduced on v8.5.8.

This display path is related to the conflict-summary support introduced by [PR #64710](https://github.com/pingcap/tidb/pull/64710), which references [issue #60621](https://github.com/pingcap/tidb/issues/60621). This report is a separate defensive-handling issue.

### 4. What is your TiDB version? (Required)

This was directly reproduced on TiDB `8.0.11-TiDB-v8.5.8` (v8.5.8). The exact output of `SELECT tidb_version()` was not preserved.

Start by locating the SHOW IMPORT JOB implementation and its successful-job display path, then trace how the persisted Summary is read for a finished job. Reproduce the NULL-summary state with the SQL shown and verify that SHOW IMPORT JOB returns the record without a nil-pointer error, with unavailable summary fields represented consistently.

Written by the indexing model from the issue text.