Fix NPE when calling os.identity().tenants().list() on Identity v2

I'm not sure this is the right thing to do, but I don't see any reference to service catalogs in the V2 API. Another take on this fix would be at https://github.com/gondor/openstack4j/blob/1.0.3/src/main/java/org/openstack4j/openstack/identity/internal/DefaultEndpointURLResolver.java#L64. Usually I prefer empty collections to checking for null.

Am I missing something or is this really a bug ?

Stack trace:

```java
java.lang.NullPointerException
	at org.openstack4j.openstack.identity.internal.DefaultEndpointURLResolver.resolveV2(DefaultEndpointURLResolver.java:64)
	at org.openstack4j.openstack.identity.internal.DefaultEndpointURLResolver.findURL(DefaultEndpointURLResolver.java:49)
	at org.openstack4j.openstack.internal.OSClientSession.getEndpoint(OSClientSession.java:179)
	at org.openstack4j.core.transport.HttpRequest$RequestBuilder.build(HttpRequest.java:373)
	at org.openstack4j.openstack.internal.BaseOpenStackService$Invocation.execute(BaseOpenStackService.java:135)
	at org.openstack4j.openstack.internal.BaseOpenStackService$Invocation.execute(BaseOpenStackService.java:131)
	at org.openstack4j.openstack.identity.internal.TenantServiceImpl.list(TenantServiceImpl.java:21)
```