Fix scheduled action panic with null event payload (#37459) (#37466)

Backport #37459 by cyphercodes

This fixes the scheduled action panic when an event payload is JSON
`null` by initializing the payload map before adding `schedule`. It also
adds regression coverage for the null-payload case.

Fixes #37447.

Co-authored-by: Rayan Salhab <r.salhab@aiyexpertsolutions.com>
Co-authored-by: cyphercodes <cyphercodes@users.noreply.github.com>
Co-authored-by: Hermes Agent (GPT-5.5) <hermes-agent@users.noreply.github.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

services/actions/schedule_tasks_test.go

@@ -22,9 +22,20 @@ func TestWithScheduleInEventPayload(t *testing.T) {
 		assert.Equal(t, "refs/heads/main", event["ref"])
 	})
 
-	t.Run("keeps empty payload", func(t *testing.T) {
+	t.Run("adds schedule to null payload", func(t *testing.T) {
+		updated := withScheduleInEventPayload("null", "37 12 5 1 2")
+
+		event := map[string]any{}
+		assert.NoError(t, json.Unmarshal([]byte(updated), &event))
+		assert.Equal(t, "37 12 5 1 2", event["schedule"])
+	})
+
+	t.Run("adds schedule to empty payload", func(t *testing.T) {
 		updated := withScheduleInEventPayload("", "37 12 5 1 2")
-		assert.Empty(t, updated)
+
+		event := map[string]any{}
+		assert.NoError(t, json.Unmarshal([]byte(updated), &event))
+		assert.Equal(t, "37 12 5 1 2", event["schedule"])
 	})
 
 	t.Run("keeps payload when schedule empty", func(t *testing.T) {