Skip fieldObjects that are not actually References

The `fieldObjects`-getter is implemented in the `PDFDocument` class, which means that the `this._localIdFactory`-property that we pass to `AnnotationFactory.create` doesn't actually exist.
The reason that this hasn't caused any bugs, that I'm aware of, is that all /Fields-entries need to be References to actually make sense.

src/core/document.js

@@ -1714,7 +1714,10 @@ class PDFDocument {
   async #collectFieldObjects(name, fieldRef, promises, annotationGlobals) {
     const { xref } = this;
 
-    const field = await xref.fetchIfRefAsync(fieldRef);
+    if (!(fieldRef instanceof Ref)) {
+      return;
+    }
+    const field = await xref.fetchAsync(fieldRef);
     if (!(field instanceof Dict)) {
       return;
     }
@@ -1731,7 +1734,7 @@ class PDFDocument {
         xref,
         fieldRef,
         annotationGlobals,
-        this._localIdFactory,
+        /* idFactory = */ null,
         /* collectFields */ true,
         /* pageRef */ null
       )