empty_as_varchar fails to work when cell borders are present

Hi,

The empty_as_varchar behaves very strangely for cells with difference formatting.

When there is empty cell present, and there no cell border, and empty_as_varchar is present, then the column type is correctly identified as VARCHAR as expected.

But when there is cell border, even though the content is empty, the column type will be identified as DOUBLE.

Here is a sample with two sheets (one with border, one without border) for reference

[reproduce.xlsx](https://github.com/user-attachments/files/24326832/reproduce.xlsx)

Start by running the Excel import against reproduce.xlsx with empty_as_varchar enabled, comparing the bordered and unbordered sheets described in the issue. Trace the type-inference path for empty cells and verify that both cases identify the column as VARCHAR, adding a regression test if the relevant test location can be found.

Written by the indexing model from the issue text.