Embedded js: fix no-sphinx docs

Page is available for sphinx projects only,
jquery was interpreting a null value as no present,
but now that we are using fetch/URLSearchParams this is being
serialized as a literal `null`.

Author: stsewd

readthedocs/core/static-src/core/js/doc-embed/footer.js

@@ -41,7 +41,9 @@ function init() {
     var get_data = {
         project: rtd['project'],
         version: rtd['version'],
-        page: rtd['page'],
+        // Page is a sphinx concept only,
+        // avoid serializing this as a literal `null` instead of empty.
+        page: rtd['page'] || "",
         theme: rtd.get_theme_name(),
     };