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/firefoxcom.js

@@ -254,17 +254,19 @@ class FirefoxComDataRangeTransport extends PDFDataRangeTransport {
   }
 }
 
-const FirefoxScripting = {
-  createSandbox(data) {
+class FirefoxScripting {
+  static createSandbox(data) {
     FirefoxCom.requestSync("createSandbox", data);
-  },
-  dispatchEventInSandbox(event, sandboxID) {
+  }
+
+  static dispatchEventInSandbox(event, sandboxID) {
     FirefoxCom.requestSync("dispatchEventInSandbox", event);
-  },
-  destroySandbox() {
+  }
+
+  static destroySandbox() {
     FirefoxCom.requestSync("destroySandbox", null);
-  },
-};
+  }
+}
 
 class FirefoxExternalServices extends DefaultExternalServices {
   static updateFindControlState(data) {