[Bug](stream-load) Stream Load crashes BE with strict_mode and all-expression `columns`

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Version

Apache Doris 4.0

### What's Wrong?

BE crashes (SIGSEGV) when a Stream Load uses `strict_mode=true` together with a `columns` header in which **every target column is assigned via an expression** (no direct-mapped column), and at least one row produces a NULL on a derived column.

### Crash location

`FileScanner::_convert_to_output_block()` in `be/src/vec/exec/scan/file_scanner.cpp`.

The strict-mode branch indexes `_src_slot_descs_order_by_dest[dest_index]` and `_dest_slot_to_src_slot_index[dest_index]` without checking their size.

### Root cause

These two members are populated only when FE sends `dest_sid_to_src_sid_without_trans`, which only happens if **at least one** target column is direct-mapped (no `=` in `columns`). When every target column uses an expression, both containers stay empty and the strict-mode branch reads them out of bounds.

### What You Expected?

Stream Load should not crash BE. The strict-mode branch should be guarded by a size / existence check on `_src_slot_descs_order_by_dest` and `_dest_slot_to_src_slot_index`, and fall through to the regular nullable check when no source-column mapping exists for a derived column.
### How to Reproduce?

**Table**

```sql
CREATE TABLE sl_min (
k BIGINT NOT NULL,
v VARCHAR(64) NULL
) ENGINE=OLAP
DUPLICATE KEY(k)
DISTRIBUTED BY HASH(k) BUCKETS 1
PROPERTIES ("replication_num" = "1");
```

**Source file** `sl_min.csv` (one row, two fields):

````
1,\N
````

**Stream Load**

```bash
curl --location-trusted -u root: \
-H "label:sl_min_$(date +%s)" \
-H "format:csv" \
-H "column_separator:," \
-H "columns:c1,c2,k=c1,v=c2" \
-H "strict_mode:true" \
-H "max_filter_ratio:0" \
-T ./sl_min.csv \
"http://:/api//sl_min/_stream_load"
```

The BE process crashes with SIGSEGV. curl reports `Warning: Binary output can mess up your terminal` because the connection is dropped mid-response.

### Anything Else?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start with be/src/vec/exec/scan/file_scanner.cpp, especially FileScanner::_convert_to_output_block() and its strict-mode branch. Reproduce the failure with the provided sl_min table, CSV row, and Stream Load curl command, then verify that all-expression columns with a NULL derived value no longer crash and use the regular nullable check when no source mapping exists.

Written by the indexing model from the issue text.