fix a few null problems and warnings

fix a few null problems and warnings

memused/cpu/loadavg don't have to be there if the rrd data does not
go all the way into the past, so instead of using ! to ignore the null
chekc, use a sensible default

if we use '??' to default to a value, there is no need
to access the last property with '.?' since if it's null it falls back
anyway. (this was only a warning from linting)

also if a value cannot be null (because of its type) we don't have
to do an explicit null check (also a warning from linting)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>