Fix https://github.com/go-gitea/gitea/issues/21666
Caused by https://github.com/go-gitea/gitea/pull/19663
Before: when install, the INTERNAL_TOKEN was always generated and saved.
But the internal token may be already there by INTERNAL_TOKEN_URI
After: INTERNAL_TOKEN_URI file must be non-empty. When install, skip
internal token generation if the token exists.