Annotations - Avoid empty value in text field when storage contains something for it (bug 1719148)

- it aims to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1719148;
  - JS can set a property for a non-rendered annotation using the annotationStorage but the other unset default properties must be used when the annotation is finally rendered;
  - so this patch just adds the properties already set in the annotationStorage to the default value.

src/display/annotation_storage.js

@@ -42,7 +42,12 @@ class AnnotationStorage {
    * @returns {Object}
    */
   getValue(key, defaultValue) {
-    return this._storage.get(key) ?? defaultValue;
+    const value = this._storage.get(key);
+    if (value === undefined) {
+      return defaultValue;
+    }
+
+    return Object.assign(defaultValue, value);
   }
 
   /**