`query unbonding-delegation` cause panic while no item in queue.

[yangyanqing]

Summary of Bug

query unbonding-delegation cause panic while no item in queue.

Version

{
  "cosmos_sdk": "0.34.6",
  "commit": "80234baf91a15dd9a7df8dca38677b66b8d148c1",
  "vendor_hash": "f60176672270c09455c01e9d880079ba36130df4f5cd89df58b6701f50b13aad",
  "build_tags": "netgo ledger",
  "go": "go version go1.12.5 darwin/amd64\n"
}

Steps to Reproduce

$ gaiacli q staking unbonding-delegation \
    cosmos16gv8hu0ctlkdr29mewtqafvzq4dzqyu32p5sdy \
    cosmosvaloper16gv8hu0ctlkdr29mewtqafvzq4dzqyu304q9ph
panic: UnmarshalBinaryLengthPrefixed cannot decode empty bytes

goroutine 1 [running]:
github.com/cosmos/cosmos-sdk/x/staking/types.MustUnmarshalUBD(0xc000ba3960, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/Users/yangyanqing/go/src/github.com/cosmos/cosmos-sdk/x/staking/types/delegation.go:172 +0x16d
github.com/cosmos/cosmos-sdk/x/staking/client/cli.GetCmdQueryUnbondingDelegation.func1(0xc000c05180, 0xc000bf71a0, 0x2, 0x2, 0x0, 0x0)
	/Users/yangyanqing/go/src/github.com/cosmos/cosmos-sdk/x/staking/client/cli/query.go:286 +0x2b0
github.com/spf13/cobra.(*Command).execute(0xc000c05180, 0xc000bf7140, 0x2, 0x2, 0xc000c05180, 0xc000bf7140)
	/Users/yangyanqing/go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:762 +0x465
github.com/spf13/cobra.(*Command).ExecuteC(0xc000194280, 0x47bbcff, 0xc000194280, 0x4b2c663)
	/Users/yangyanqing/go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:852 +0x2ec
github.com/spf13/cobra.(*Command).Execute(...)
	/Users/yangyanqing/go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:800
github.com/tendermint/tendermint/libs/cli.Executor.Execute(0xc000194280, 0x4ef5fe8, 0x2, 0xc000af0800)
	/Users/yangyanqing/go/pkg/mod/github.com/tendermint/tendermint@v0.31.5/libs/cli/setup.go:89 +0x3c
main.main()
	/Users/yangyanqing/go/src/github.com/cosmos/cosmos-sdk/cmd/gaia/cmd/gaiacli/main.go:110 +0x818

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• Appropriate labels applied
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