being hit by the problem of plasmashell using way too much CPU even when doing "nothing", I found
the description of the QSG_RENDER_LOOP env variable to be settable to basic, which solves the problem.
However, even though I set the var in ~/.config/plasma-workspace/env/KDE5.sh plasmashell (and only this process)
always ends up having the var set to an EMPTY string (I check this via /proc/PIC/environ).

I don't know from where RendererSettings is initialized, but I assume that the QString inside it is
not null but empty, so isNull() == false but isEmpty() == true

With this patch, QSG_RENDER_LOOP=basic is now correct in plasmashell's env after a login.