Fix 500-error when no value policy in self-reg

It should be fixed more seriously by avoiding exception-masking
loadObject method. This is only a quick hack.

(cherry picked from commit 0473679)

gui/admin-gui/src/main/java/com/evolveum/midpoint/web/page/login/PageSelfRegistration.java

@@ -688,6 +688,10 @@ private <O extends ObjectType> String generateNonce(NonceCredentialsPolicyType n
 		if (noncePolicy != null && noncePolicy.getValuePolicyRef() != null) {
 			PrismObject<ValuePolicyType> valuePolicy = WebModelServiceUtils.loadObject(ValuePolicyType.class,
 					noncePolicy.getValuePolicyRef().getOid(), PageSelfRegistration.this, task, result);
+			if (valuePolicy == null) {
+				LOGGER.error("Nonce cannot be generated, as value policy {} cannot be fetched", noncePolicy.getValuePolicyRef().getOid());
+				throw new ObjectNotFoundException("Nonce cannot be generated");         // no more information (security); TODO implement more correctly
+			}
 			policy = valuePolicy.asObjectable();
 		}