Avoid unnecessary parsing, in Page.GetStructTree, when no structTree is available (PR 13221 follow-up)

It's obviously (a bit) more efficient to return early in `Page.getStructTree`, rather than trying to first "parse" an *empty* structTree-root.

*Somehow I didn't think of this yesterday, but this feels like a much better solution overall; sorry about the churn here!*

src/core/document.js

@@ -454,7 +454,13 @@ class Page {
     const structTreeRoot = await this.pdfManager.ensureCatalog(
       "structTreeRoot"
     );
-    return this.pdfManager.ensure(this, "_parseStructTree", [structTreeRoot]);
+    if (!structTreeRoot) {
+      return null;
+    }
+    const structTree = await this.pdfManager.ensure(this, "_parseStructTree", [
+      structTreeRoot,
+    ]);
+    return structTree.serializable;
   }
 
   /**

src/core/struct_tree.js

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

src/core/worker.js

@@ -743,15 +743,9 @@ class WorkerMessageHandler {
     });
 
     handler.on("GetStructTree", function wphGetStructTree(data) {
-      const pageIndex = data.pageIndex;
+      return pdfManager.getPage(data.pageIndex).then(function (page) {
-      return pdfManager
+        return pdfManager.ensure(page, "getStructTree");
-        .getPage(pageIndex)
+      });
-        .then(function (page) {
-          return pdfManager.ensure(page, "getStructTree");
-        })
-        .then(function (structTree) {
-          return structTree.serializable;
-        });
     });
 
     handler.on("FontFallback", function (data) {