[cli] fix crash on requesting non-existing block

[olegrok]

Before this patch simple request could crash CLI:

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

Closes #675

[Un1oR]

Damn, how ugly this train of boolean flags looks in WinAPI style 🙈 In C++ we had a rule not to use bool parameters in functions other than trivial setters, but to have an enum with two values. I'm not sure how suitable this is for Go with the absence of enums as such and weak typing 😞