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,
+                };
               })
           );
         }

test/pdfs/.gitignore

@@ -366,6 +366,7 @@
 !issue1045.pdf
 !issue5010.pdf
 !issue10339_reduced.pdf
+!issue15557.pdf
 !issue4934.pdf
 !issue4650.pdf
 !issue6721_reduced.pdf

test/test_manifest.json

@@ -646,6 +646,13 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "issue15557",
+       "file": "pdfs/issue15557.pdf",
+       "md5": "5f024a42acf27f3eadc4c7418e8f4e42",
+       "rounds": 1,
+       "type": "eq",
+       "annotations": true
+    },
     {  "id": "hmm-pdf",
        "file": "pdfs/hmm.pdf",
        "md5": "e08467e60101ee5f4a59716e86db6dc9",