Simplify setting the GlobalWorkerOptions default values (PR 9480 follow-up)

There's really no need for these "complicated" default value assignments, since `GlobalWorkerOptions` is a local variable at this point, and this is rather a case of too much copy-and-paste.
Note that years ago, when all options were set using a global `PDFJS` object, it's possible that options had been set (from the outside) *before* the object had been properly initialized; see e.g. a89071bdef/src/display/global.js

src/display/worker_options.js

@@ -27,14 +27,7 @@
 /** @type {GlobalWorkerOptionsType} */
 const GlobalWorkerOptions = Object.create(null);
 
-GlobalWorkerOptions.workerPort =
+GlobalWorkerOptions.workerPort = null;
-  GlobalWorkerOptions.workerPort === undefined
+GlobalWorkerOptions.workerSrc = "";
-    ? null
-    : GlobalWorkerOptions.workerPort;
-
-GlobalWorkerOptions.workerSrc =
-  GlobalWorkerOptions.workerSrc === undefined
-    ? ""
-    : GlobalWorkerOptions.workerSrc;
 
 export { GlobalWorkerOptions };