The mozilla-central [method `PdfDataListener.readData`](https://searchfox.org/mozilla-central/rev/893a8f062ec6144c84403fbfb0a57234418b89cf/toolkit/components/pdfjs/content/PdfStreamConverter.jsm#207-210) can return `null`, hence it seems like a very good idea to update `PDFDataTransportStream._onReceiveData` to handle that gracefully since the current code will throw in that case. Also, improves the JSDocs for the `PDFDataRangeTransport` class in the API.