add explicit nullable type

[SergeiMV]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.63%. Comparing base (1665a72) to head (bc26ae7).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #319      +/-   ##
============================================
+ Coverage     93.25%   93.63%   +0.37%     
  Complexity      124      124              
============================================
  Files            14       14              
  Lines           326      314      -12     
============================================
- Hits            304      294      -10     
+ Misses           22       20       -2     

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