fix(core): support provide empty avatar src (#9454)

Co-authored-by: Anna Zhukova <[email protected]>

projects/core/pipes/fallback-src/fallback-src.pipe.ts

@@ -14,7 +14,7 @@ export class TuiFallbackSrcPipe implements PipeTransform {
     public transform(src: string, fallback: string): Observable<string> {
         return fromEvent(this.el, 'error', {capture: true}).pipe(
             map(() => fallback),
-            startWith(src),
+            startWith(src || fallback),
         );
     }
 }