Fix docs that cause unchecked assignment and NPE

docs/manual/src/docs/asciidoc/_includes/reactive/oauth2/resource-server.adoc

@@ -464,10 +464,11 @@ static class GrantedAuthoritiesExtractor
         implements Converter<Jwt, Collection<GrantedAuthority>> {
 
     public Collection<GrantedAuthority> convert(Jwt jwt) {
-        Collection<String> authorities = (Collection<String>)
-                jwt.getClaims().get("mycustomclaim");
+        Collection<?> authorities = (Collection<?>)
+                jwt.getClaims().getOrDefault("mycustomclaim", Collections.emptyList());
 
         return authorities.stream()
+                .map(Object::toString)
                 .map(SimpleGrantedAuthority::new)
                 .collect(Collectors.toList());
     }

docs/manual/src/docs/asciidoc/_includes/servlet/oauth2/oauth2-resourceserver.adoc

@@ -496,10 +496,11 @@ static class GrantedAuthoritiesExtractor
         implements Converter<Jwt, Collection<GrantedAuthority>> {
 
     public Collection<GrantedAuthority> convert(Jwt jwt) {
-        Collection<String> authorities = (Collection<String>)
-                jwt.getClaims().get("mycustomclaim");
+        Collection<?> authorities = (Collection<?>)
+                jwt.getClaims().getOrDefault("mycustomclaim", Collections.emptyList());
 
         return authorities.stream()
+                .map(Object::toString)
                 .map(SimpleGrantedAuthority::new)
                 .collect(Collectors.toList());
     }