fix(datepicker): showing dot in high contrast mode after popup has closed (#12099)

Fixes the datepicker's popup showing a 1x1 pixel dot in high contrast mode, after the calendar has been closed. The dot comes from the outline that we add to the popup which collapses after we detach the overlay.

src/lib/datepicker/calendar-body.scss

@@ -68,7 +68,7 @@ $mat-calendar-body-cell-content-size: 100% - $mat-calendar-body-cell-content-mar
 
 
 @include cdk-high-contrast {
-  .mat-datepicker-popup,
+  .mat-datepicker-popup:not(:empty),
   .mat-calendar-body-selected {
     outline: solid 1px;
   }