Jonas Jenwald dc19965d78 Slightly re-factor how the BaseViewer/PDFThumbnailViewer handle page labels internally, to make the null default value clearer
Currently it's not *immediately* clear from the code itself, unless you look at the definition of `this._pageLabels`, that the default value is `null`.[1]
We can improve this, and also reduce the amount of code, by using modern ECMAScript features such as optional chaining and nullish coalescing.

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[1] Keep in mind that an *empty* string is actually a valid page label, according to the PDF specification.
2021-02-05 17:50:07 +01:00
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