[enhancement] no network at a glance

[mattia-b89]

I think would be a good enhancement to better handle the case with no network;

currently,

• if there is no network activity, speeds are zero, as it should be;
• but if there is no network at all, speeds are even now zero;

even if it is true because no network means no network activity,
it bring us to err: you see 0KB/s and you think "there is no network activity",
but in that moment could be no network at all;

We can change the behaviour when there is no network at all (common issue with laptops, i.e. weak WiFi signal...) from showing 0KB/s to ---KB/s, or no network icon, or different network icon, or something else