set strictNullChecks to true in tsconfig.json and fix errors

[oltolm]

Another cleanup. I set strictNullChecks to true in tsconfig.json and the fixed all the errors.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 2.91262% with 100 lines in your changes are missing coverage. Please review.

Project coverage is 20.29%. Comparing base (1b9f13e) to head (417a462).

Files	Patch %	Lines
src/frontend/extension.ts	0.00%	26 Missing :warning:
src/backend/mi2/mi2.ts	0.00%	24 Missing :warning:
src/mibase.ts	0.00%	22 Missing :warning:
src/gdb.ts	0.00%	5 Missing :warning:
src/lldb.ts	0.00%	5 Missing :warning:
src/mago.ts	0.00%	5 Missing :warning:
src/backend/linux/console.ts	0.00%	4 Missing :warning:
src/backend/mi2/mi2lldb.ts	0.00%	4 Missing :warning:
src/backend/mi2/mi2mago.ts	0.00%	2 Missing :warning:
src/backend/mi_parse.ts	50.00%	0 Missing and 2 partials :warning:
... and 1 more

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #422      +/-   ##
==========================================
- Coverage   20.47%   20.29%   -0.18%     
==========================================
  Files          14       14              
  Lines        1807     1818      +11     
  Branches      392      404      +12     
==========================================
- Hits          370      369       -1     
- Misses       1392     1402      +10     
- Partials       45       47       +2     

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[oltolm]

I fixed everything.

[oltolm]

Is there still something to do to get it accepted?