Ensure that useWorkerFetch fallback value is always a boolean

If either of the factory-urls are missing or invalid, the fallback value would currently become `useWorkerFetch === null`.
While that is obviously a falsy value, which means that the code still works as intended, we should ensure that this is consistent.

src/display/api.js

@@ -331,15 +331,17 @@ function getDocument(src = {}) {
     typeof src.useWorkerFetch === "boolean"
       ? src.useWorkerFetch
       : (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) ||
-        (CMapReaderFactory === DOMCMapReaderFactory &&
+        !!(
+          CMapReaderFactory === DOMCMapReaderFactory &&
           StandardFontDataFactory === DOMStandardFontDataFactory &&
           WasmFactory === DOMWasmFactory &&
           cMapUrl &&
           standardFontDataUrl &&
           wasmUrl &&
           isValidFetchUrl(cMapUrl, document.baseURI) &&
           isValidFetchUrl(standardFontDataUrl, document.baseURI) &&
-          isValidFetchUrl(wasmUrl, document.baseURI));
+          isValidFetchUrl(wasmUrl, document.baseURI)
+        );
 
   // Parameters only intended for development/testing purposes.
   const styleElement =