`nil block` crashes on attempt to request non-existing block

[olegrok]

nil block 0x0004c3c56fde2450c25c9f0501588655be1483e083014f0e6ad4fa617c68432b
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x125a4be]

goroutine 1 [running]:
github.com/NilFoundation/nil/nil/services/rpc/jsonrpc.(*DebugRPCBlock).Decode(0x0)
	github.com/NilFoundation/nil/nil/services/rpc/jsonrpc/types.go:182 +0x3e
github.com/NilFoundation/nil/nil/services/rpc/jsonrpc.(*DebugRPCBlock).DecodeSSZ(0xc000312000?)
	github.com/NilFoundation/nil/nil/services/rpc/jsonrpc/types.go:199 +0x13
github.com/NilFoundation/nil/nil/services/cliservice.(*Service).FetchDebugBlock(0xc00048c460, 0x1, {0x16cada0?, 0xc0004742e0?}, 0x0, 0x0, 0x0)
	github.com/NilFoundation/nil/nil/services/cliservice/block.go:46 +0x7d
github.com/NilFoundation/nil/nil/cmd/nil/internal/block.runCommand(0xc000506300?, {0xc000474f30, 0x1, 0x1a3e358?})
	github.com/NilFoundation/nil/nil/cmd/nil/internal/block/block.go:44 +0xaa
github.com/spf13/cobra.(*Command).execute(0xc00044d508, {0xc000474f00, 0x1, 0x1})
	github.com/spf13/cobra@v1.9.1/command.go:1015 +0xa94
github.com/spf13/cobra.(*Command).ExecuteC(0xc00044c908)
	github.com/spf13/cobra@v1.9.1/command.go:1148 +0x40c
github.com/spf13/cobra.(*Command).Execute(...)
	github.com/spf13/cobra@v1.9.1/command.go:1071
main.(*RootCommand).Execute(0xc000482400?)
	github.com/NilFoundation/nil/nil/cmd/nil/main.go:144 +0x16
main.main()
	github.com/NilFoundation/nil/nil/cmd/nil/main.go:120 +0x254