Don't throw exception if related object can't be found

[jameskleeh]

If a document has a reference to another domain class and the document for that domain class cannot found, it should return null instead of throwing an exception. Add as a configuration option to prevent breaking existing behavior

[graemerocher]

So the issue with this is we cannot know whether the association exists or not so we create proxies to that association. The proxy is not null and when the proxy is loaded if the association doesn't exist we throw an exception. The only way to know if something is null is to perform a query to check, but doing that for each association will hurt performance. I see no solution for this issue as it stands.