Fixed ArgumentNullException in call to GetFull

Calling `GetFull` when connection to elasticsearch server is unavailable results in an `ArgumentNullException`. This is different from the behavior of other methods which typically return connection error information as part of the `IReponse.ConnectionStatus` property.

A question concerning this issue:
Is checking the `ConnectionStatus` the proper strategy for determining if the ES server was unreachable? Would `ElasticClient` throwing an exception of some sort not be a more appropriate strategy? An exception would allow for a high level exception handler rather than having to check the `ConnectionStatus` response every place `ElasticClient` used. Just trying to understand the reasoning.

Thanks!