fix: hide unavailable sequence title in course breadcrumbs (#28028)

When a sequence title was unavailable for a user (e.g. before starting a timed
exam), the `None` value was displayed in course breadcrumbs. This hides it in
such cases and ensures that a trailing breadcrumb separator is not displayed.

common/static/sass/edx-pattern-library-shims/_breadcrumbs.scss

@@ -57,5 +57,10 @@
     @include rtl {
       @include transform(rotateY(180deg));
     }
+
+    // Hide a trailing separator.
+    &:last-child {
+      display: none;
+    }
   }
 }

lms/templates/courseware/courseware.html

@@ -237,7 +237,9 @@ ${HTML(fragment.foot_html())}
                                 </span>
                                 <span class="icon fa fa-angle-right" aria-hidden="true"></span>
                             % endif
-                            <span class="nav-item nav-item-sequence">${sequence_title}</span>
+                            % if sequence_title:
+                                <span class="nav-item nav-item-sequence">${sequence_title}</span>
+                            % endif
                         </div>
                     </div>
                 </nav>