Update all `BaseViewer.getPageView`/`PDFThumbnailViewer.getThumbnail` call-sites, in `web/app.js`, to check the returned value properly
Update all `BaseViewer.getPageView`/`PDFThumbnailViewer.getThumbnail` call-sites, in `web/app.js`, to check the returned value properly

Given how those are used, there *should* not be any situation in which e.g. `undefined` is ever returned. However, actually checking that the pageView/thumbnail is defined cannot hurt.

Also, re-factor `webViewerPageRendered` slightly since the `pageView` is no longer unconditionally necessary after the previous patches; note in particular that the thumbnails will only be updated when the sidebar *and* the thumbnailView is visible.

Finally, fixes a bug in `webViewerPageChanging` whereby an empty string would not be treated as a valid pageLabel and instead be replaced by `null`.

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