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/block.go

@@ -90,7 +90,7 @@ func GetBlocking(ctx context.Context, blockerID, blockeeID int64) (*Blocking, er
 		return nil, err
 	}
 	if len(blocks) == 0 {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return blocks[0], nil
 }

models/user/email_address.go

@@ -215,7 +215,7 @@ func GetEmailAddressByID(ctx context.Context, uid, id int64) (*EmailAddress, err
 	if has, err := db.GetEngine(ctx).ID(id).Get(email); err != nil {
 		return nil, err
 	} else if !has {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return email, nil
 }

models/user/list.go

@@ -48,7 +48,7 @@ func (users UserList) GetTwoFaStatus(ctx context.Context) map[int64]bool {
 
 func (users UserList) loadTwoFactorStatus(ctx context.Context) (map[int64]*auth.TwoFactor, error) {
 	if len(users) == 0 {
-		return nil, nil
+		return nil, nil //nolint:nilnil // returns nil when there are no users
 	}
 
 	userIDs := users.GetUserIDs()

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])