[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.

src/core/struct_tree.js

@@ -328,6 +328,10 @@ class StructTreePage {
       }
       nodeToSerializable(child, root);
     }
+
+    if (root.children.length === 0) {
+      return null;
+    }
     return root;
   }
 }