fix(persistence): port poll DAO null-user fix and chunked size guard from #2731

[VelikovPetar]

Linear: FLU-
Github Issue: #

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Description of the pull request

Ports the review fixes from #2731 (commit 15af740) onto v9. These were applied during the merge-master-to-v10 PR but never landed on the v9 branch directly.

• PollDao: switch the user side of the polls LEFT JOIN users read from readTable(users) to readTableOrNull(users) in both _pollFromJoinRow and getPollsByIds, and make userEntity nullable in _buildPoll. Without this, reading a poll whose creator user isn't in the local cache crashes (the LEFT JOIN legitimately returns no user row).
• chunked(): throw ArgumentError when size <= 0 instead of looping forever / producing nonsense chunks.
• Adds a unit test file for chunked() covering the empty/single/even/partial/default-size cases plus the new zero/negative guards. All 7 tests pass locally (flutter test test/src/db/query_utils_test.dart).

Screenshots / Videos

NA — persistence-layer change.

Before	After
getPollById / getPollsByIds throws when the creator user row is absent.	Returns the poll with createdBy: null.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (v9@fa471c5). Learn more about missing BASE report.

Additional details and impacted files

@@          Coverage Diff          @@
##             v9    #2752   +/-   ##
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  Coverage      ?   65.98%           
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  Files         ?      425           
  Lines         ?    26952           
  Branches      ?        0           
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  Hits          ?    17784           
  Misses        ?     9168           
  Partials      ?        0           

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