chore: btclightclient Hashes gRPC panic

[RafilxTenfen]

Cantina AI BABY-6

x/btclightclient/keeper/grpc_query.go:37 does len(req.Pagination.Key) without nil check. Any unauthenticated caller can panic a node (recovered by the query server handling) by sending a gRPC request without the pagination field. The MainChain handler in the same file correctly handles nil pagination.