fix(for-of): handle null values when getting observed nodes

Author: viktorkombov

projects/igniteui-angular/directives/src/directives/for-of/for_of.directive.spec.ts

@@ -1496,7 +1496,7 @@ export class TestIgxForOfDirective<T> extends IgxForOfDirective<T> {
         return super.getBorder(node, dimension);
     }
 
-    public testGetViewObservedNode(view: any): Element {
+    public testGetViewObservedNode(view: any): Element | null {
         return super.getViewObservedNode(view);
     }
 

projects/igniteui-angular/directives/src/directives/for-of/for_of.directive.ts

@@ -533,14 +533,14 @@ export class IgxForOfDirective<T, U extends T[] = T[]> extends IgxForOfToken<T,U
      * hold no element at all, or be torn down before its content is rendered -
      * in which case there is nothing to observe and null is returned.
      */
-    protected getViewObservedNode(view: EmbeddedViewRef<any>): Element {
+    protected getViewObservedNode(view: EmbeddedViewRef<any> | null | undefined): Element | null {
         const rootNodes = view?.rootNodes ?? [];
-        const elementNode = rootNodes.find(node => node?.nodeType === Node.ELEMENT_NODE);
+        const elementNode = rootNodes.find(node=> node?.nodeType === Node.ELEMENT_NODE);
         const candidate = elementNode ?? rootNodes[0]?.nextElementSibling;
         return candidate?.nodeType === Node.ELEMENT_NODE ? candidate : null;
     }
 
-    protected subscribeToViewObserver(target: Element) {
+    protected subscribeToViewObserver(target: Element | null) {
         if (!target) {
             return;
         }