do not show twistie for a childless compressed node

[weinand]

Testing #101009

An uncompressed debug session tree shows twisties only where it makes sense:

The corresponding compressed tree shows a twistie despite the fact that it is really the "leaf" from the uncompressed tree:

[connor4312]

I was about to open an issue for this. I can reproduce this by having a parent session that spawns and unspawns a child, specifically running this script via launch config or the debug terminal: https://github.com/microsoft/vscode-js-debug/blob/master/demos/node/parent.js

[isidorn]

Well I automatically expand all parent sessions so the compressed state would be immediatly visible..
I also introduced a check to not auto expand the same session twice so that user can explicitlyt collapse something.
I just change the logic now that we allow to auto collapse a session if it has a new child element - what seem to be the case here.