Simplify the newRefs computation in the "SaveDocument"-handler in the worker-thread

- Let the `Page.save`-method filter out "empty" entries, similar to the `Page._parsedAnnotations`-getter, since that on its own already simplifies the "SaveDocument"-handler a tiny bit.

 - The existing `reduce` and `concat` construction isn't exactly a wonder of readability :-)
   Thanks to modern JavaScript features it should be possible to replace all of this with `Array.prototype.flat()` instead, which at least to me feels a lot easier to understand.

src/core/document.js

@@ -349,7 +349,9 @@ class Page {
         );
       }
 
-      return Promise.all(newRefsPromises);
+      return Promise.all(newRefsPromises).then(function (newRefs) {
+        return newRefs.filter(newRef => !!newRef);
+      });
     });
   }