Test explicitely if cpuPercent is defined. If not, the key 'cpu_perce…

…nt' didn't exist in the 'data'. Previously, the update would not only be skipped if the key didn't exist, but _also_ if the cpuPercent was zero, which would result in the CPU usage only being updated _during_ execution of a cell (when CPU usage is non-zero), and it would never return to zero afterwards

jupyter_resource_usage/static/main.js

@@ -74,7 +74,7 @@ define([
 
                 // Handle CPU display
                 var cpuPercent = data['cpu_percent'];
-                if (cpuPercent) {
+                if (cpuPercent !== undefined) {
                     // Remove hide CSS class if the metrics API gives us a CPU percent to display
                     $('#jupyter-resource-usage-display-cpu').removeClass('jupyter-resource-usage-hide');
                     var maxCpu = data['cpu_count'];