nil pointer dereference in TestLeasingDisconnectedGet

[mitake]

I got below error during CI of #9817
Probably gRPC side problem but opening this issue for sharing.

=== RUN   TestLeasingDisconnectedGet
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x60 pc=0xb5c7ca]
goroutine 23206 [running]:
go.etcd.io/etcd/vendor/google.golang.org/grpc.(*csAttempt).sendMsg(0xc00017cb00, 0x1341340, 0xc000389ad0, 0xc0055dae79, 0x5, 0x5, 0xc0055dae90, 0x10, 0x10, 0xc0055dae90, ...)
	/go/src/go.etcd.io/etcd/vendor/google.golang.org/grpc/stream.go:713 +0x14a
go.etcd.io/etcd/vendor/google.golang.org/grpc.(*clientStream).SendMsg.func2(0xc00017cb00, 0xb5b88f, 0xc000303e50)
	/go/src/go.etcd.io/etcd/vendor/google.golang.org/grpc/stream.go:637 +0x186
go.etcd.io/etcd/vendor/google.golang.org/grpc.(*clientStream).withRetry(0xc002468360, 0xc000303e50, 0xc00164faa8, 0xc0055dae79, 0x0)
	/go/src/go.etcd.io/etcd/vendor/google.golang.org/grpc/stream.go:530 +0x43d
go.etcd.io/etcd/vendor/google.golang.org/grpc.(*clientStream).SendMsg(0xc002468360, 0x1341340, 0xc000389ad0, 0x0, 0x0)
	/go/src/go.etcd.io/etcd/vendor/google.golang.org/grpc/stream.go:643 +0x777
go.etcd.io/etcd/etcdserver/etcdserverpb.(*watchWatchClient).Send(0xc000389ac0, 0xc000389ad0, 0x0, 0x1e49f58)
	/go/src/go.etcd.io/etcd/etcdserver/etcdserverpb/rpc.pb.go:3697 +0x6b
go.etcd.io/etcd/clientv3.(*watchGrpcStream).run(0xc0016d72b0)
	/go/src/go.etcd.io/etcd/clientv3/watch.go:546 +0xd9b
created by go.etcd.io/etcd/clientv3.(*watcher).newWatcherGrpcStream
	/go/src/go.etcd.io/etcd/clientv3/watch.go:277 +0x58c
FAIL	go.etcd.io/etcd/clientv3/integration	129.885s

[stale]

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