Use `Promise.prototype.finally` in the `PDFRenderingQueue.renderView` method
Use `Promise.prototype.finally` in the `PDFRenderingQueue.renderView` method

Since [`finally`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/finally) is now supported through `src/shared/compatibility.js`, the temporary variable is no longer necessary.

Also, this patch fixes the inconsistent return type of `PDFRenderingQueue.getHighestPriority` when no pages/thumbnails are visible.

caremerge/pdf.js,macroplant/pdf.js,nawawi/pdf.js,mainegreen/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,mozilla/pdf.js,mukulmishra18/pdf.js,Snuffleupagus/pdf.js,macroplant/pdf.js,timvandermeij/pdf.js,nawawi/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,xavier114fch/pdf.js,mukulmishra18/pdf.js,mainegreen/pdf.js,mozilla/pdf.js,caremerge/pdf.js,xavier114fch/pdf.js,macroplant/pdf.js,nawawi/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js