SessionUser protection against nil pointer dereference (#21581)

Backport #21358 

`SessionUser` should be protected against passing `sess` = `nil` to
avoid

```
PANIC: runtime error: invalid memory address or nil pointer dereference
```

in


https://github.com/go-gitea/gitea/pull/18452/files#diff-a215b82aadeb8b4c4632fcf31215dd421f804eb1c0137ec6721b980136e4442aR69

after upgrade from gitea v1.16 to v1.17.

Related: #18452

services/auth/session.go

@@ -39,6 +39,10 @@ func (s *Session) Verify(req *http.Request, w http.ResponseWriter, store DataSto
 
 // SessionUser returns the user object corresponding to the "uid" session variable.
 func SessionUser(sess SessionStore) *user_model.User {
+	if sess == nil {
+		return nil
+	}
+
 	// Get user ID
 	uid := sess.Get("uid")
 	if uid == nil {