我有一个使用img标签来显示图像的代码。问题是当图像src为空时，我会得到一个错误，说404错误。我如何解决这个问题。我将在下面分享我的代码。

<ng-container [ngSwitch]="item.itemTitle" id="container">
                          
                          <img class="img thumbnail" (click)="openDialogImg(myTemplate)"
                            *ngSwitchCase="
                              ['Graph1', 'Graph2'].includes(item.itemTitle)
                                ? item.itemTitle
                                : !item.itemTitle
                            "
                            [src]="_sanitizer.bypassSecurityTrustResourceUrl(item.itemValue)"
                            alt="Graph"
                          /> 
                          <!-- this template is invisible. It will be shown in the popup -->
                         <ng-template #myTemplate>                          
                           <!-- this is a big popup image.  -->
                           <img [src]="_sanitizer.bypassSecurityTrustResourceUrl(item.itemValue)" style="height:100%; width:100%">
                           <button id = "x" mat-button (click)="myDialogRef?.close()">X</button>
                         </ng-template>                        
                          <span *ngSwitchDefault>{{ item.itemValue }}</span>
                        </ng-container>

当graph1和graph2图像数据为空时，我得到404错误