Fix a couple of edge-cases in PDFViewerApplication._initializeJavaScript (PR 12432 follow-up)

- Return early in `PDFViewerApplication._initializeJavaScript` for PDF documents without any `fieldObjects`, which is the vast majority of all documents, to prevent errors when trying to parse a non-existent object.

 - Similar to the other `PDFViewerApplication._initialize*` methods, ignore the `fieldObjects` if the document was closed before the data resolved.

 - Fix the JSDoc comment for the `generateRandomStringForSandbox` helper function, since there's currently a bit too much copy-and-paste going on :-)

 - Change `FirefoxScripting` to a class with static methods, which is consistent with the surrounding code in `web/firefoxcom.js`.

web/ui_utils.js

@@ -1010,8 +1010,8 @@ function getActiveOrFocusedElement() {
 /**
  * Generate a random string which is not define somewhere in actions.
  *
- * @param {WaitOnEventOrTimeoutParameters}
- * @returns {Promise} A promise that is resolved with a {WaitOnType} value.
+ * @param {Object} objects - The value returned by `getFieldObjects` in the API.
+ * @returns {string} A unique string.
  */
 function generateRandomStringForSandbox(objects) {
   const allObjects = Object.values(objects).flat(2);