ElasticNetCVLearner: Fix parameter alphas

[janezd]

Issue

ElasticNetCVLearner requires int or an array, and no longer accepts None.

Description of changes

Set it to the default value, 100. This duplicates the value of n_alpha, which we need to keep for compatibility

Includes

• Code changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.02%. Comparing base (441070e) to head (43b5dad).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7294      +/-   ##
==========================================
- Coverage   89.03%   89.02%   -0.01%     
==========================================
  Files         336      336              
  Lines       74433    74433              
==========================================
- Hits        66273    66267       -6     
- Misses       8160     8166       +6     

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