Enable TypeScript compiler strict null checks [WPB-22420]

[screendriver]

WPB-22420 [Web] General maintenance ticket for PR merges

• guarantee that enabling strict null checks does not change message, calling, invitation, or notification behavior
• keep nullability explicit and readable so invariants remain obvious at call sites
• maintain repository-wide confidence by keeping both test behavior and type contracts aligned

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 Security Hotspot

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 31.55792% with 514 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.89%. Comparing base (fcb5443) to head (b83a7fb).
⚠️ Report is 3 commits behind head on dev.

Files with missing lines	Patch %	Lines
...epositories/conversation/ConversationRepository.ts	19.23%	105 Missing and 21 partials ⚠️
...itories/conversation/ConversationRoleRepository.ts	41.02%	19 Missing and 4 partials ⚠️
...rc/script/repositories/conversation/EventMapper.ts	37.83%	8 Missing and 15 partials ⚠️
...pt/repositories/cryptography/CryptographyMapper.ts	25.80%	13 Missing and 10 partials ⚠️
...src/script/repositories/client/ClientRepository.ts	5.26%	18 Missing ⚠️
...ipt/repositories/conversation/MessageRepository.ts	0.00%	17 Missing ⚠️
apps/webapp/src/script/util/debugUtil.ts	0.00%	17 Missing ⚠️
...app/src/script/repositories/entity/Conversation.ts	65.11%	13 Missing and 2 partials ⚠️
...repositories/conversation/ConversationCellState.ts	12.50%	12 Missing and 2 partials ⚠️
...ositories/conversation/ConversationStateHandler.ts	0.00%	14 Missing ⚠️
... and 66 more

Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #21203      +/-   ##
==========================================
- Coverage   45.01%   44.89%   -0.13%     
==========================================
  Files        1701     1701              
  Lines       44219    44562     +343     
  Branches     9113     9320     +207     
==========================================
+ Hits        19905    20005     +100     
- Misses      22095    22266     +171     
- Partials     2219     2291      +72     

Flag	Coverage Δ
app_webapp	43.09% <31.09%> (-0.13%)	⬇️
lib_api_client	51.34% <ø> (ø)
lib_core	58.53% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...bapp/src/script/auth/module/action/ClientAction.ts	44.68% <ø> (ø)
...s/webapp/src/script/auth/page/SingleSignOnForm.tsx	61.58% <100.00%> (ø)
apps/webapp/src/script/auth/util/urlUtil.ts	36.36% <ø> (ø)
...cript/page/MainContent/panels/Collection/utils.tsx	81.81% <100.00%> (+6.81%)	⬆️
...rc/script/repositories/calling/videoGridHandler.ts	90.00% <100.00%> (+12.72%)	⬆️
...app/src/script/repositories/client/ClientMapper.ts	93.54% <100.00%> (+0.69%)	⬆️
...script/repositories/connection/connectionEntity.ts	100.00% <100.00%> (ø)
...script/repositories/connection/connectionMapper.ts	100.00% <100.00%> (ø)
...pp/src/script/repositories/entity/message/Asset.ts	68.42% <100.00%> (+1.75%)	⬆️
.../script/repositories/entity/message/CallMessage.ts	90.90% <ø> (ø)
... and 99 more

... and 12 files with indirect coverage changes

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[github-actions]

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🧪 Playwright Test Summary

• ✅ Passed: 14
• ❌ Failed: 0
• ⏭ Skipped: 0
• 🔁 Flaky: 1
• 📊 Total: 15
• ⏱ Total Runtime: 162.6s (~ 2 min 43 sec)

specs/CriticalFlow/channelsCall-TC-8755.spec.ts (❌ 0 failed, ⚠️ 1 flaky)

• ⚠️ Calls in channels with device switch and screenshare (tags: TC-8755, crit-flow-web)