CYB-327 thor keys:lti_new should not give error if there's no API key

  * The thor task to generate LTI keys prints out the new keys and the API key,
     if there is no API key it was trying to concatenate to nil

[CYB-327 thor keys:lti_new should not give error if there's no API key](https://jira.ets.berkeley.edu/jira/browse/CYB-327)