fix: allow viewing Actions run triggered by deleted user (#12271)

Fixes #9371.  Manually reproduced and tested by setting `action_run.triggering_user_id` to a non-existent user ID.  Manually tested that runs can be cancelled in this state as well.

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### Tests for Go changes

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
  - [x] `make pr-go` before pushing

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12271
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

models/actions/run.go

@@ -153,7 +153,9 @@ func (run *ActionRun) LoadAttributes(ctx context.Context) error {
 
 	if run.TriggerUser == nil {
 		u, err := user_model.GetPossibleUserByID(ctx, run.TriggerUserID)
-		if err != nil {
+		if user_model.IsErrUserNotExist(err) {
+			u = user_model.NewGhostUser()
+		} else if err != nil {
 			return err
 		}
 		run.TriggerUser = u

models/actions/run_test.go

@@ -19,9 +19,6 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestGetRunBefore(t *testing.T) {
-}
-
 func TestSetConcurrencyGroup(t *testing.T) {
 	run := ActionRun{}
 	run.SetConcurrencyGroup("abc123")
@@ -686,3 +683,12 @@ jobs:
 	assert.Zero(t, insertedJobs[1].TaskID)
 	assert.Equal(t, StatusWaiting, insertedJobs[1].Status)
 }
+
+func TestActionRunLoadAttributes(t *testing.T) {
+	run := &ActionRun{
+		RepoID:        10,
+		TriggerUserID: 1000,
+	}
+	require.NoError(t, run.LoadAttributes(t.Context()))
+	assert.Equal(t, "ghost", run.TriggerUser.LowerName)
+}