Don't show prefix if no elapsed time

Marlin/src/lcd/dogm/status_screen_DOGM.cpp

           lcd_put_wchar(PROGRESS_BAR_X, EXTRAS_BASELINE, SHOW_REMAINING_TIME_PREFIX);
           lcd_put_u8str(estimation_x_pos, EXTRAS_BASELINE, estimation_string);
         }
-        else {
+        else if (elapsed_string[0] != '\0'){
           lcd_put_wchar(PROGRESS_BAR_X, EXTRAS_BASELINE, ELAPSED_TIME_PREFIX);
           lcd_put_u8str(elapsed_x_pos, EXTRAS_BASELINE, elapsed_string);
         }