Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

models/user/user.go

@@ -1193,7 +1193,7 @@ func (eum *EmailUserMap) GetByEmail(email string) *User {
 
 func GetUsersByEmails(ctx context.Context, emails []string) (*EmailUserMap, error) {
 	if len(emails) == 0 {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil when there are no emails to look up
 	}
 
 	needCheckEmails := make(container.Set[string])