Fix/pop in cat refit

[blondered]

• Fixed PopularInCategoryModel refit behaviour and cross-validate compatibility
• Fixed PopularInCategoryModel empty category interactions behaviour
• Added tests Closes https://github.com/MobileTeleSystems/RecTools/issues/162

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (9b3992e) to head (2ecedcb). Report is 68 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #163 +/- ## =========================================== Coverage 100.00% 100.00% =========================================== Files 45 55 +10 Lines 2242 3304 +1062 =========================================== + Hits 2242 3304 +1062 ```

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