fix keycloak public key

Description

Description of the MR

Merge Request Checklists

• I have already covered the unit testing
• Logical data model has been updated
• Deployment documentation has been updated

Optional Additional notes

src/main/java/eu/europa/ec/simpl/usersroles/services/impl/KeycloakServiceImpl.java

@@ -58,8 +58,9 @@ public class KeycloakServiceImpl implements KeycloakService {
     public String getPublicKey() {
         return getAppRealm().keys().getKeyMetadata().getKeys().stream()
                 .filter(k -> Objects.equals(k.getUse(), SIG))
-                .findFirst()
                 .map(KeysMetadataRepresentation.KeyMetadataRepresentation::getPublicKey)
+                .filter(Objects::nonNull)
+                .findFirst()
                 .orElseThrow(NotFoundException::new);
     }
 }