test(zalo): cover empty getUpdates response + extract poll timeout headroom

Follow-up to #924:
- Add TestGetUpdates_EmptyResultReturnsNilSlice to cover the no-events
  polling branch (API returns {} when no pending updates).
- Extract magic 7s into pollTimeoutHeadroom const for readability.

internal/channels/zalo/zalo.go

@@ -24,11 +24,12 @@ import (
 )
 
 const (
-	defaultPollTimeout = 30
-	maxTextLength      = 2000
-	defaultMediaMaxMB  = 5
-	pollErrorBackoff   = 5 * time.Second
-	pairingDebounce    = 60 * time.Second
+	defaultPollTimeout  = 30
+	maxTextLength       = 2000
+	defaultMediaMaxMB   = 5
+	pollErrorBackoff    = 5 * time.Second
+	pairingDebounce     = 60 * time.Second
+	pollTimeoutHeadroom = 7 * time.Second
 )
 
 // apiBase is the Zalo Bot API root. Declared as a variable so tests can
@@ -494,7 +495,7 @@ func (c *Channel) getUpdates(timeout int) ([]zaloUpdate, error) {
 		"timeout": timeout,
 	}
 
-	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(timeout+7)*time.Second)
+	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(timeout)*time.Second+pollTimeoutHeadroom)
 	defer cancel()
 
 	result, err := c.callAPIWith(ctx, c.pollClient, "getUpdates", params)

internal/channels/zalo/zalo_test.go

@@ -165,6 +165,26 @@ func TestGetUpdates_ParsesSingleUpdate(t *testing.T) {
 	}
 }
 
+// TestGetUpdates_EmptyResultReturnsNilSlice verifies that when the Zalo API
+// returns an empty update object (no pending events), getUpdates returns nil
+// instead of a slice with a zero-valued element — the poll loop treats it as
+// "nothing to dispatch" without invoking processUpdate.
+func TestGetUpdates_EmptyResultReturnsNilSlice(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte(`{"ok":true,"result":{}}`))
+	}))
+	defer srv.Close()
+
+	ch := newTestChannel(t, srv.URL)
+	updates, err := ch.getUpdates(1)
+	if err != nil {
+		t.Fatalf("getUpdates: %v", err)
+	}
+	if updates != nil {
+		t.Errorf("updates = %+v, want nil (empty result object)", updates)
+	}
+}
+
 // TestGetUpdates_UnmarshalError verifies a malformed result surfaces an error.
 func TestGetUpdates_UnmarshalError(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {