nullsafe check for refresh

openid-connect-common/src/main/java/org/mitre/oauth2/model/ClientDetailsEntity.java

@@ -250,7 +250,11 @@ public class ClientDetailsEntity implements ClientDetails {
      */
 	@Transient
     public boolean isAllowRefresh() {
+		if (grantTypes != null) {
 			return getAuthorizedGrantTypes().contains("refresh_token");
+		} else {
+			return false; // if there are no grants, we can't be refreshing them, can we?
+		}
     }
 
 	@Basic