I too would like a better solution as sometimes setting custom folding doesnt work in my workflow.
My work-around is to whack a comment before the whitespace that I don't want folded:

a topic
. a note
 ? a question regarding the note
  - a to do item
 . some more details
. another note
 . more details
#END a topic

another topic
...

->

+a topic
#END a topic

another topic

In Practical use: I add that trailing comment to all my class-def snippets (etc) so they automatically preserve the whitespace after that definition without needing custom folding regions
:-(

If someone knows how to tell VSCode to simply never cull empty lines, my problem would be solved without needing my work-around.