update undefined to null

Co-authored-by: Maksim Sadym <[email protected]>

index.bs

@@ -2274,7 +2274,7 @@ To <dfn>get the browsing context info</dfn> given |context|,
    <code>browsingContext.Info</code> production with the <code>context</code>
    field set to |context id|, the <code>parent</code> field set to |parent id|
    if |is root| is <code>true</code>, or unset otherwise, the <code>url</code>
-   field set to |url|, the <code>userContext</code> field set to |user context|
+   field set to |url|, the <code>userContext</code> field set to |user context| if it’s not null, or omitted otherwise,
    and the <code>children</code> field set to |child infos|.
 
 1. Return |context info|.