[VSCO] Fix JSON returned by VSCO (#7821)

* [VSCO] Fix JSON returned by VSCO

The JSON returned contains multiple `undefined` values that aren't valid
JSON, changing these values to `null` fixes the issue.

* [VSCO] Make JSON fix more targeted

Change only occurrences that are values of a key-value

gallery_dl/extractor/vsco.py

@@ -79,7 +79,7 @@ class VscoExtractor(Extractor):
     def _extract_preload_state(self, url):
         page = self.request(url, notfound=self.subcategory).text
         return util.json_loads(text.extr(page, "__PRELOADED_STATE__ = ", "<")
-                               .replace('"prevPageToken":undefined,', ''))
+                               .replace('":undefined', '":null'))
 
     def _pagination(self, url, params, token, key, extra=None):
         headers = {