Fix NPE in Kotlin docs example

Closes gh-14634

docs/modules/ROOT/pages/servlet/exploits/csrf.adoc

@@ -876,7 +876,7 @@ class SpaCsrfTokenRequestHandler : CsrfTokenRequestAttributeHandler() {
         delegate.handle(request, response, csrfToken)
     }
 
-    override fun resolveCsrfTokenValue(request: HttpServletRequest, csrfToken: CsrfToken): String {
+    override fun resolveCsrfTokenValue(request: HttpServletRequest, csrfToken: CsrfToken): String? {
         /*
          * If the request contains a request header, use CsrfTokenRequestAttributeHandler
          * to resolve the CsrfToken. This applies when a single-page application includes