Only run tabbing logic on docs that contain blocks

Summary:
Builds upon #14212
Closes https://github.com/facebook/react-native/pull/14213

Differential Revision: D5149113

Pulled By: hramos

fbshipit-source-id: 99684bbb3bcae0e6061641373c6af7446729d7bd

website/src/react-native/js/scripts.js

@@ -202,6 +202,10 @@
   }
 
   function guessPlatformAndOS() {
+    if (!document.querySelector('block')) {
+      return;
+    }
+
     // If we are coming to the page with a hash in it (i.e. from a search, for example), try to get
     // us as close as possible to the correct platform and dev os using the hashtag and block walk up.
     var foundHash = false;