Fix/analytics key errors

[alexskr]

Addresses:

• /analytics Endpoint Errors Out When Year is Not Present in Data #232

[codecov]

Codecov Report

Attention: Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.

Project coverage is 81.08%. Comparing base (7a46e09) to head (1985ac3).

Files with missing lines	Patch %	Lines
lib/ontologies_linked_data/models/ontology.rb	14.28%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #233   +/-   ##
========================================
  Coverage    81.08%   81.08%           
========================================
  Files           74       74           
  Lines         5250     5250           
========================================
  Hits          4257     4257           
  Misses         993      993           

Flag	Coverage Δ
unittests	81.08% <14.28%> (ø)

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