Handle missing SQL Server deadlock sessions

[joelmarcotte]

What does this PR do?

Returns an empty deadlock result when no usable XE session exists. Adds a regression test for the missing-session path.

Motivation

The query method returned None, which _create_deadlock_rows then attempted to iterate and raised TypeError.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[cit-pr-commenter-54b7da]

evalya-impact-summary

evalya impact analysis

Impact analysis: 0 selected, 0 skipped (of 0 test tasks)
Publish tasks:   2 (always emitted)
Diff (2 files):
  sqlserver/datadog_checks/sqlserver/deadlocks.py
  sqlserver/tests/test_deadlocks.py

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[dd-octo-sts]

Validation Report

All 21 validations passed.

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[datadog-datadog-prod-us1-2]

 

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 91.25% (+2.41%)

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