Excel Extension fails to load all data even though stop_at_empty = false

### What happens?

When there is an empty column in the sheet, duckdb stops at that column.

### To Reproduce

Code:
```
import duckdb

print(
duckdb.sql(
"SELECT * FROM read_xlsx('D:/test.xlsx', all_varchar=true, header=false, stop_at_empty=false, ignore_errors = true);"
).fetchall()
)
```

Content of `D:/test.xlsx`:
| | A | B | C |
|---|---|---|---|
| 1 | X | | |
| 2 | | | |
| 3 | | | X |

Result:
`[('X',), (None,)]`

Expected:
`[('X', None, None), (None, None, None), (None, None, X)]`, or at least `[('X',), (None,), (None,)]`

### OS:

Windows x86_64

### DuckDB Package Version:

1.3.2

### Python Version:

3.13.7

### Full Name:

Luna Yao

### Affiliation:

Self

### What is the latest build you tested with? If possible, we recommend testing with the latest nightly build.

I have tested with a stable release

### Did you include all relevant data sets for reproducing the issue?

Yes

### Did you include all code required to reproduce the issue?

- [x] Yes, I have

### Did you include all relevant configuration to reproduce the issue?

- [x] Yes, I have

首先，针对复现的输入 D:/test.xlsx 运行提供的 Python 查询，并检查 Excel 扩展使用的 read_xlsx 入口点。当空列分隔有数据的单元格时，将结果与预期的三列输出进行比较，然后添加覆盖测试以表明 stop_at_empty=false 会保留后续数据，并运行相关测试。

由索引模型根据 Issue 内容生成。