api: variables GetItems method causes panic if variable does not exist

[shoenig]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x10c8a61]

goroutine 1 [running]:
github.com/hashicorp/nomad/api.(*Variables).GetItems(0xc000d7eeb8?, {0x7fffef67316b?, 0x1?}, 0x4b79476?)
	/home/shoenig/Work/go/nomad/api/variables.go:174 +0xe1
github.com/traefik/traefik/v2/pkg/provider/acme.get[...](0xc0013cda40, {0x7fffef67316b, 0xc}, {0x4b80e20, 0x7})
	/home/shoenig/Work/go/traefik/pkg/provider/acme/nomad_store.go:128 +0x77
github.com/traefik/traefik/v2/pkg/provider/acme.(*NomadStore).GetAccount(0xc0013cda40, {0xc000bce8ee, 0x2})
	/home/shoenig/Work/go/traefik/pkg/provider/acme/nomad_store.go:66 +0x2bc

This should return an error, but we'll have to fix #16234 first