fix(material/table): th with nbsp; instead of span

Author: forsti0506

src/components-examples/material/table/table-expandable-rows/table-expandable-rows-example.html

@@ -6,7 +6,7 @@
     <td mat-cell *matCellDef="let element"> {{element[column]}} </td>
   </ng-container>
   <ng-container matColumnDef="expand">
-    <th mat-header-cell *matHeaderCellDef><span aria-label="row actions"></span></th>
+    <th mat-header-cell *matHeaderCellDef aria-label="row actions">&nbsp;</th>
     <td mat-cell *matCellDef="let element"> 
       <button mat-icon-button aria-label="expand row" (click)="(expandedElement = expandedElement === element ? null : element); $event.stopPropagation()">
         <mat-icon *ngIf="expandedElement !== element">keyboard_arrow_down</mat-icon> 

src/components-examples/material/table/table-sticky-columns/table-sticky-columns-example.html

@@ -27,7 +27,7 @@
 
     <!-- Star Column -->
     <ng-container matColumnDef="star" stickyEnd>
-      <th mat-header-cell *matHeaderCellDef><span aria-label="row actions"></span></th>
+      <th mat-header-cell *matHeaderCellDef aria-label="row actions">&nbsp;</th>
       <td mat-cell *matCellDef="let element">
         <mat-icon>more_vert</mat-icon>
       </td>