Skip Promise.all in PDFViewerApplication._parseHashParameters unless actually necessary

Given that only two debugging hash parameters (i.e. `disableWorker` and `pdfBug`) will make this method asynchronous, we can avoid what's most of the time is an unnecessary `Promise.all` invocation.

web/app.js

@@ -385,6 +385,9 @@ const PDFViewerApplication = {
       AppOptions.set("locale", hashParams.locale);
     }
 
+    if (waitOn.length === 0) {
+      return undefined;
+    }
     return Promise.all(waitOn).catch(reason => {
       console.error(`_parseHashParameters: "${reason.message}".`);
     });