fix: preserve empty parallel executor context

- hoist the empty executor guard out of the inner closure to match NewPipelineExecutor
- preserve canceled context semantics when no executors are provided
- add coverage for the empty executor case

act/common/executor.go

@@ -97,6 +97,12 @@ func NewErrorExecutor(err error) Executor {
 
 // NewParallelExecutor creates a new executor from a parallel of other executors
 func NewParallelExecutor(parallel int, executors ...Executor) Executor {
+	if len(executors) == 0 {
+		return func(ctx context.Context) error {
+			return ctx.Err()
+		}
+	}
+
 	return func(ctx context.Context) error {
 		work := make(chan Executor, len(executors))
 		errs := make(chan error, len(executors))

act/common/executor_test.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestNewWorkflow(t *testing.T) {
@@ -119,6 +120,19 @@ func TestNewParallelExecutor(t *testing.T) {
 	assert.NoError(errSingle)
 }
 
+func TestNewParallelExecutorEmpty(t *testing.T) {
+	assert := assert.New(t)
+
+	ctx := context.Background()
+	require.NoError(t, NewParallelExecutor(2)(ctx))
+
+	canceledCtx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	err := NewParallelExecutor(2)(canceledCtx)
+	assert.ErrorIs(err, context.Canceled)
+}
+
 func TestNewParallelExecutorFailed(t *testing.T) {
 	assert := assert.New(t)