hst/config: make identifier omitempty

This is an optional field. Serialise it as such.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/hakurei/print_test.go

@@ -712,7 +712,6 @@ func TestPrintPs(t *testing.T) {
     "instance": "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
     "pid": 48879,
     "shim_pid": 51966,
-    "id": "",
     "enablements": {
       "wayland": true,
       "pulse": true