Remove the "DocProgress" loaded fallback from the getPdfManager function

Falling back to use the `loaded` byteLength if the server `contentLength` is unknown doesn't make a lot of sense, since it'd lead to the `onProgress` callback reporting `percent === 100` repeatedly while the document is loading despite that being obviously wrong.
Instead we'll now report `percent === NaN` in that case, thus showing the indeterminate progressBar, which seems more correct if the `contentLength` is unknown.

Please note that this code-path is normally not even reached, since streaming is enabled by default (applies e.g. to the Firefox PDF Viewer).

src/core/worker.js

@@ -275,7 +275,7 @@ class WorkerMessageHandler {
           if (!fullReader.isStreamingSupported) {
             handler.send("DocProgress", {
               loaded,
-              total: Math.max(loaded, fullReader.contentLength || 0),
+              total: fullReader.contentLength,
             });
           }