Fix nil pointer dereference in adrules api

[WASDetchan]

See issue #5748. Closes issue #5748.
Previous behavior:

$ ./xray api adrules -s=127.0.0.1:8080 empty_routing.json
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x11330dc]

goroutine 1 [running]:
github.com/xtls/xray-core/main/commands/all/api.executeAddRules(0x2271c80, {0x2557ff0ca030, 0x2, 0x2})
	github.com/xtls/xray-core/main/commands/all/api/rules_add.go:66 +0x3dc
github.com/xtls/xray-core/main/commands/base.Execute()
	github.com/xtls/xray-core/main/commands/base/execute.go:64 +0x59f
main.main()
	github.com/xtls/xray-core/main/main.go:22 +0x1a5

New behavior:

$ ./xray api adrules -s=127.0.0.1:8080 ../empty_routing.json
failed to add routing rule: config did not have "routing" field