fix(appHtmlAttr): cannot remove html attribute

usage:
- `attributes: {hidden: null}` removes attribute `hidden`

Author: xidedix

projects/coreui/angular/src/lib/shared/layout/layout.directive.ts

@@ -169,6 +169,8 @@ export class HtmlAttributesDirective implements OnInit {
   }
 
   private setAttrib(key, value) {
-    this.renderer.setAttribute(this.el.nativeElement, key, value );
+    value !== null ?
+      this.renderer.setAttribute(this.el.nativeElement, key, value ) :
+      this.renderer.removeAttribute(this.el.nativeElement, key);
   }
 }