HC: progressbar style fix

Reset styling on the progress node when trough node has the `empty'
styleclass.

See https://bugzilla.gnome.org/show_bug.cgi?id=774695

gtk/theme/HighContrast/_common.scss

@@ -2484,6 +2484,7 @@ progressbar {
       border-radius: 0;
     }
   }
+  trough.empty progress { all: unset; } // make the progress indicator disappear when the fraction is 0
 }
 
 /*************

gtk/theme/HighContrast/gtk-contained-inverse.css

@@ -2529,6 +2529,8 @@ progressbar progress {
   progressbar progress.osd {
     border-width: 0;
     border-radius: 0; }
+progressbar trough.empty progress {
+  all: unset; }
 
 /*************
  * Level Bar *

gtk/theme/HighContrast/gtk-contained.css

@@ -2536,6 +2536,8 @@ progressbar progress {
   progressbar progress.osd {
     border-width: 0;
     border-radius: 0; }
+progressbar trough.empty progress {
+  all: unset; }
 
 /*************
  * Level Bar *