[api-minor] Let PDFPageProxy.getStructTree return null, rather than an empty structTree, for documents without any accessibility data (PR 13171 follow-up)

This is first of all consistent with existing API-methods, where we return `null` when the data in question doesn't exist. Secondly, it should also be (slightly) more efficient since there's less dummy-data that we need to transfer between threads.
Finally, this prevents us from adding an empty/unnecessary span to *every* single page even in documents without any structure tree data.

web/pdf_page_view.js

@@ -619,6 +619,9 @@ class PDFPageView {
         this.eventBus._off("textlayerrendered", this._onTextLayerRendered);
         this._onTextLayerRendered = null;
         this.pdfPage.getStructTree().then(tree => {
+          if (!tree) {
+            return;
+          }
           const treeDom = this.structTreeLayer.render(tree);
           treeDom.classList.add("structTree");
           this.canvas.appendChild(treeDom);