Add error info if no user can fork the repo (#25820)

Before: (Owner list is empty)

![image](https://github.com/go-gitea/gitea/assets/18380374/dbe87bfe-14ed-4997-8cb2-5c1308232a70)

After: (Disable the button and show the error info)

![image](https://github.com/go-gitea/gitea/assets/18380374/a7e4c315-67ab-408b-88f2-c554076ce87a)

options/locale/locale_en-US.ini

@@ -935,6 +935,7 @@ fork_from = Fork From
 already_forked = You've already forked %s
 fork_to_different_account = Fork to a different account
 fork_visibility_helper = The visibility of a forked repository cannot be changed.
+fork_no_valid_owners = This repository can not be forked because there are no valid owners.
 use_template = Use this template
 clone_in_vsc = Clone in VS Code
 download_zip = Download ZIP

routers/web/repo/pull.go

@@ -174,6 +174,12 @@ func getForkRepository(ctx *context.Context) *repo_model.Repository {
 		ctx.Data["ContextUser"] = ctx.Doer
 	} else if len(orgs) > 0 {
 		ctx.Data["ContextUser"] = orgs[0]
+	} else {
+		msg := ctx.Tr("repo.fork_no_valid_owners")
+		ctx.Data["Flash"] = ctx.Flash
+		ctx.Flash.Error(msg)
+		ctx.Data["CanForkRepo"] = false
+		return nil
 	}
 
 	return forkRepo