Describe the bug
If user does not have a DAT and goes to Download Access Token, it raises a 500 internal server error.

To Reproduce
Steps to reproduce the behavior:
Setup a new system with oAuth2 login.
Configure dat.method = Uuid.
User logs in and select Data Access Token in dropdown.
User clicks "Download token".
It opens a new page with error.

Expected behavior
If not DAT exists, it should create one and share it with user.

Additional context
https://github.com/cBioPortal/cbioportal/blob/master/src/main/java/org/cbioportal/legacy/service/impl/UuidDataAccessTokenServiceImpl.java#L102

public DataAccessToken getDataAccessToken(String username) {
    List<DataAccessToken> allDataAccessTokens =
        dataAccessTokenRepository.getAllDataAccessTokensForUsername(username);
    DataAccessToken newestDataAccessToken = allDataAccessTokens.get(allDataAccessTokens.size() - 1);
    return newestDataAccessToken;
  }

It does not check if allDataAccessTokens len is 0.