Prefer declaration over initialization in grpc query handlers

[DimitrisJim]

Summary

Taken from #3346 (comment), there's a preference for declaring and not initializing variables in the code base.

Problem Definition

Be consistent with the use of declarations where applicable, prefer declarations over declaration + initialization in the code base. This appears to exist in most of the grpc handlers, some examples (non exhaustive):

• In the client handler for getting the consensus state.
• In the connection handler for getting the connections.
• In the channel handler for getting the channels.

Changing these usually also requires the tests be changed in order to expect the nil slice in the cases where nothing is returned.

Proposal

Change occurrences of initialization into declaration in the query handlers (and the tests that depend on the response).

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