Ignore null-chars when using structTree-data in the viewer

Testing the `tagged_stamp.pdf` document locally in the viewer, I noticed that e.g. the /Alt entry for the StampAnnotation contains "Secondary text for stamp\u0000".
Elsewhere in the viewer we're skipping null-chars and it's easy enough to do that in the `StructTreeLayerBuilder` class as well. (Note that we generally let the API itself return the data as-is.)

web/struct_tree_layer_builder.js

@@ -13,6 +13,8 @@
  * limitations under the License.
  */
 
+import { removeNullCharacters } from "./ui_utils.js";
+
 const PDF_ROLE_TO_HTML_ROLE = {
  // Document level structure types
  Document: null, // There's a "document" role, but it doesn't make sense here.
@@ -102,13 +104,13 @@ class StructTreeLayerBuilder {
  #setAttributes(structElement, htmlElement) {
  const { alt, id, lang } = structElement;
  if (alt !== undefined) {
- htmlElement.setAttribute("aria-label", alt);
+ htmlElement.setAttribute("aria-label", removeNullCharacters(alt));
  }
  if (id !== undefined) {
  htmlElement.setAttribute("aria-owns", id);
  }
  if (lang !== undefined) {
- htmlElement.setAttribute("lang", lang);
+ htmlElement.setAttribute("lang", removeNullCharacters(lang));
  }
  }