LibreOffice Calc and Pivot table with empty cells

I want to process survey results with LibreOffice Calc (for example with data coming from LimeSurvey).

Since some questions were optional, some cells are empty (like A6):

I want to answer questions like:

• How many people have given the answer 2?
• How many people did not respond?

So, I've created a pivot table with Insert → Pivot:

but... I was not able to answer the question "How many people did not respond?" (see B6).

Actual bug:

• the cell B6 counting empty cells is empty

Expected result:

• the cell B6 counting empty cells should be 1 (sum of empty cells = 1)

To fix this problem I have a workaround consisting in creating a "temporary table" and replacing empty cells with something weird like § EMPTY VALUE § to then be able to analyze empty values with my pivot table.

How do you handle empty cells in LibreOffice Calc with a pivot table?

Tested on LibreOffice 6.4.7 on Debian GNU/Linux but I can reproduce on Ubuntu 20.04 Focal Fossa and on other operating systems.

Thank you so much!