Let Annotation._collectActions return null when no actions are present

Rather than returning an *empty* Object[1] we should be returning `null` instead, since that's consistent with existing API-functionality.
To avoid having to *manually* track if the Object is empty, this patch also introduces a small helper function to check its size.

src/core/annotation.js

@@ -25,6 +25,7 @@ import {
   escapeString,
   getModificationDate,
   isString,
+  objectSize,
   OPS,
   shadow,
   stringToPDFString,
@@ -1460,18 +1461,20 @@ class WidgetAnnotation extends Annotation {
     if (dict.has("AA")) {
       const additionalActions = dict.get("AA");
       for (const key of additionalActions.getKeys()) {
-        if (key in AnnotationActionEventType) {
-          const actionDict = additionalActions.getRaw(key);
-          const parents = new RefSet();
-          const list = [];
-          this._collectJS(actionDict, xref, list, parents);
-          if (list.length > 0) {
-            actions[AnnotationActionEventType[key]] = list;
-          }
+        const action = AnnotationActionEventType[key];
+        if (!action) {
+          continue;
+        }
+        const actionDict = additionalActions.getRaw(key);
+        const parents = new RefSet();
+        const list = [];
+        this._collectJS(actionDict, xref, list, parents);
+        if (list.length > 0) {
+          actions[action] = list;
         }
       }
     }
-    // Collect the Action if any (we may have one on pushbutton)
+    // Collect the Action if any (we may have one on pushbutton).
     if (dict.has("A")) {
       const actionDict = dict.get("A");
       const parents = new RefSet();
@@ -1481,7 +1484,7 @@ class WidgetAnnotation extends Annotation {
         actions.Action = list;
       }
     }
-    return actions;
+    return objectSize(actions) > 0 ? actions : null;
   }
 
   getFieldObject() {

src/shared/util.js

@@ -585,6 +585,10 @@ function string32(value) {
   );
 }
 
+function objectSize(obj) {
+  return Object.keys(obj).length;
+}
+
 // Ensures that the returned Object has a `null` prototype.
 function objectFromEntries(iterable) {
   return Object.assign(Object.create(null), Object.fromEntries(iterable));
@@ -1040,6 +1044,7 @@ export {
   isString,
   isSameOrigin,
   createValidAbsoluteUrl,
+  objectSize,
   objectFromEntries,
   IsLittleEndianCached,
   IsEvalSupportedCached,