Don't render the flash-complete progress event as 0%

The connection library signals completion by calling the progress
callback with an undefined value. Since the connection library
v1.0.0-beta.0 upgrade (e6b4a7d) we rendered that as 0%, so the
progress bar visibly jumped backwards while the dialog stayed open
during post-flash reinit/reset (most noticeable after partial flash).
Hold the bar at its last value until the dialog closes instead.

Author: microbit-matt-hillsdon

src/project/project-actions.tsx

@@ -530,7 +530,9 @@ export class ProjectActions {
       const progress = (stage: ProgressStage, value?: number) => {
         const isPartial = stage === ProgressStage.PartialFlashing;
         const isFlashing = isPartial || stage === ProgressStage.FullFlashing;
-        if (isFlashing) {
+        // An undefined value for a flashing stage signals completion; hold the
+        // bar at its last value until the dialog closes rather than showing 0.
+        if (isFlashing && value !== undefined) {
           this.dialogs.progress({
             header: flashingCode,
             body: isPartial ? undefined : firstFlashNotice,