Inconsistent and Incorrect conversion of text "NA" to unknown in the File widget

**What's wrong?**

**On importing standard CSV data file with a category column , it sometimes converts the category text "NA" to "unknown" ie "?" , but not always.**

My current workaround is to not use NA but rename it NX and it works fine.

**How can we reproduce the problem?**

[Filebug.ows.zip](https://github.com/biolab/orange3/files/15366492/Filebug.ows.zip)

[Test2.csv](https://github.com/biolab/orange3/files/15366493/Test2.csv)

Instructions:
1 - check the contents of the test2.csv to seee the innocuous use of "NA" as a category value in the "PD" column.

2 - Now open the filebug.ows in Orange3, open the File widget to upload the Test2.csv data.

3 - Open the Data Table to see how the data has been uploaded, scroll down to where you expect to see the "NA" text value and notice that precisely the rows which use the "NA" value have been modified and the "NA" replaced with "?"

The behaviour is inconsistent because if you create a smaller table using the category values "NA", the import works fine.

**What's your environment?**

- Operating system:
- Orange version:
- How you installed Orange:
I am using Orange 3.36 on MAC OS Sanoma V 14.4.1 installed with the Orange DMG

Reproduce the report using Test2.csv and Filebug.ows.zip in the File widget, then inspect the imported values in Data Table. Compare the larger table with the smaller table described in the report and trace how the category value "NA" becomes "?". Done means CSV imports preserve "NA" consistently as a category value.

Written by the indexing model from the issue text.