Add isRunning check to getCertainty

Author: microbit-robert

pxtextension.ts

@@ -150,6 +150,10 @@ namespace ml {
   //% parts="v2"
   //% help=github:machine-learning/docs/ml_get_event_certainty
   export function getCertainty(event: MlEvent): number {
+    if (!isRunning()) {
+      startRunning();
+      return 0;
+    }
     const eventValue = event.eventValue;
     if (eventValue <= 1) {
       // `unknown` can't have a certainty.