Review fixes: record cancelled requests as 499; empty usage is an array

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

internal/admin/admin_ops.go

@@ -141,6 +141,9 @@ func (hx *handler) usageGet(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusInternalServerError, "usage: "+err.Error())
 		return
 	}
+	if rows == nil {
+		rows = []store.UsageRow{}
+	}
 	if r.Header.Get("Accept") == "text/plain" {
 		writeUsageTable(w, rows)
 		return

internal/admin/usage_empty_test.go

@@ -0,0 +1,31 @@
+package admin_test
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"git.wntrmute.dev/kyle/crossbar/internal/store"
+)
+
+// An empty usage table is an empty JSON array, not null: clients iterate it.
+func TestUsageEmptyIsAnArray(t *testing.T) {
+	r := newRig(t)
+	for _, q := range []string{"/_crossbar/usage", "/_crossbar/usage?by=host", "/_crossbar/usage?by=model&since=1h"} {
+		rec := r.do(t, "GET", q, "")
+		if rec.Code != 200 {
+			t.Fatalf("%s: %d", q, rec.Code)
+		}
+		if strings.TrimSpace(rec.Body.String()) != "[]" {
+			t.Errorf("%s: body %q, want []", q, rec.Body.String())
+		}
+		var rows []store.UsageRow
+		if err := json.Unmarshal(rec.Body.Bytes(), &rows); err != nil || rows == nil || len(rows) != 0 {
+			t.Errorf("%s: decoded %v %v, want an empty non-nil slice", q, rows, err)
+		}
+	}
+	rec := r.do(t, "GET", "/_crossbar/usage?by=route", "", "Accept", "text/plain")
+	if rec.Code != 200 || !strings.Contains(rec.Body.String(), "key") {
+		t.Errorf("text form with no rows must still print the header: %d %q", rec.Code, rec.Body.String())
+	}
+}