Handle case data without classifications

[gbp]

A NoMethodError occurred in background:

  undefined method `[]' for nil:NilClass
  gems/icasework-0.1.1/lib/icasework/case.rb:33:in `case_data'

-------------------------------
Backtrace:
-------------------------------

  gems/icasework-0.1.1/lib/icasework/case.rb:33:in `case_data'
  gems/icasework-0.1.1/lib/icasework/case.rb:15:in `block in where'
  gems/icasework-0.1.1/lib/icasework/case.rb:15:in `map'
  gems/icasework-0.1.1/lib/icasework/case.rb:15:in `where'
  app/models/case_management/icasework.rb:39:in `published_requests'
  app/services/disclosure_log.rb:29:in `import'
  app/services/disclosure_log.rb:19:in `import!'
  app/workers/import_disclosure_log_worker.rb:11:in `perform'

Seems to be due to cases not being classified within iCasework. We should handle this gracefully.