Ensure that Page.getOperatorList handles Annotation parsing errors correctly (issue 15557)

*Fixes a regression from PR 15246, sorry about that!*

The return value of all `Annotation.getOperatorList` methods was changed in PR 15246, however I missed updating the error code-path in `Page.getOperatorList` which thus breaks all operatorList-parsing for pages with corrupt Annotations.

src/core/document.js

@@ -486,7 +486,11 @@ class Page {
                   "getOperatorList - ignoring annotation data during " +
                     `"${task.name}" task: "${reason}".`
                 );
-                return null;
+                return {
+                  opList: null,
+                  separateForm: false,
+                  separateCanvas: false,
+                };
               })
           );
         }