fix(accordion): don't crash for panels with no content

Fixes #2250

Closes #2251

src/accordion/accordion.spec.ts

@@ -261,6 +261,20 @@ describe('ngb-accordion', () => {
     expect(childTitle).not.toContain('parent title');
   });
 
+  it('should not crash for an empty accordion', () => {
+    const fixture = createTestComponent('<ngb-accordion></ngb-accordion>');
+    expect(getPanels(fixture.nativeElement).length).toBe(0);
+  });
+
+  it('should not crash for panels without content', () => {
+    const fixture =
+        createTestComponent('<ngb-accordion activeIds="open_me"><ngb-panel id="open_me"></ngb-panel></ngb-accordion>');
+    const panelsContent = getPanelsContent(fixture.nativeElement);
+
+    expect(panelsContent.length).toBe(1);
+    expect(panelsContent[0].textContent.trim()).toBe('');
+  });
+
   it('should have the appropriate content', () => {
     const fixture = TestBed.createComponent(TestComponent);
     fixture.detectChanges();

src/accordion/accordion.ts

@@ -124,7 +124,7 @@ export interface NgbPanelChangeEvent {
         </div>
         <div id="{{panel.id}}" role="tabpanel" [attr.aria-labelledby]="panel.id + '-header'"
              class="card-body collapse {{panel.isOpen ? 'show' : null}}" *ngIf="!destroyOnHide || panel.isOpen">
-             <ng-template [ngTemplateOutlet]="panel.contentTpl.templateRef"></ng-template>
+             <ng-template [ngTemplateOutlet]="panel.contentTpl?.templateRef"></ng-template>
         </div>
       </div>
     </ng-template>