Fix nil panic if repo doesn't exist (go-gitea#32501)

fix  go-gitea#32496

(cherry picked from commit 985e2a8)

Author: wxiaoguang

models/activities/action.go

@@ -250,6 +250,9 @@ func (a *Action) GetActDisplayNameTitle(ctx context.Context) string {
 // GetRepoUserName returns the name of the action repository owner.
 func (a *Action) GetRepoUserName(ctx context.Context) string {
 	a.loadRepo(ctx)
+	if a.Repo == nil {
+		return "(non-existing-repo)"
+	}
 	return a.Repo.OwnerName
 }
 
@@ -262,6 +265,9 @@ func (a *Action) ShortRepoUserName(ctx context.Context) string {
 // GetRepoName returns the name of the action repository.
 func (a *Action) GetRepoName(ctx context.Context) string {
 	a.loadRepo(ctx)
+	if a.Repo == nil {
+		return "(non-existing-repo)"
+	}
 	return a.Repo.Name
 }