fix: add null type to null initialization variables

[miettal]

$ npx tsc --strict
Found 3646 errors in 210 files.
↓
Found 3666 errors in 188 files.

This change adds null type to null initialization variables. This change increases the number of typescript strict mode errors, but this typing is based on the fact of the code.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 3.57143% with 54 lines in your changes missing coverage. Please review.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #2701      +/-   ##
=============================================
+ Coverage      56.03%   56.05%   +0.02%     
- Complexity      8041     8047       +6     
=============================================
  Files           2059     2059              
  Lines          87513    87513              
  Branches        6418     6418              
=============================================
+ Hits           49032    49043      +11     
+ Misses         36797    36786      -11     
  Partials        1684     1684              

[miettal]

rebased.