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