Prevent errors if PDFDocumentProxy.saveDocument is called without the annotationStorage parameter (PR 12241 follow-up)

Obviously it doesn't make sense to call that method without providing an `AnnotationStorage`-instance, however we should ensure that doing so won't cause errors.
Hence we need to check that `annotationStorage` is actually defined, before attempting to call its `resetModified` method.

src/display/api.js

@@ -2544,7 +2544,9 @@ class WorkerTransport {
         filename: this._fullReader ? this._fullReader.filename : null,
       })
       .finally(() => {
-        annotationStorage.resetModified();
+        if (annotationStorage) {
+          annotationStorage.resetModified();
+        }
       });
   }