Tweak the useWorkerFetch default value checks (PR 15879 follow-up)

Currently we accidentally accept `cMapUrl` and `standardFontDataUrl` parameters that are empty strings or `null`, since e.g. `new URL(null, document.baseURI)` doesn't throw, when validating the `useWorkerFetch` parameter via the `isValidFetchUrl` helper function.
Please note that we are currently failing gracefully in this case, as intended, however the warning-messages printed in the console are perhaps less helpful without this patch.

src/display/api.js

@@ -316,6 +316,8 @@ function getDocument(src) {
       : (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) ||
         (CMapReaderFactory === DOMCMapReaderFactory &&
           StandardFontDataFactory === DOMStandardFontDataFactory &&
+          cMapUrl &&
+          standardFontDataUrl &&
           isValidFetchUrl(cMapUrl, document.baseURI) &&
           isValidFetchUrl(standardFontDataUrl, document.baseURI));
   const canvasFactory =