fix: emit first markdown detail line when rPrior is null

[bengler9]

Fixes #4035

Summary

• Fix MARKDOWN output dropping the first detail line when multiple rows share the same Finding and the first row is the earliest markdown-eligible result
• Use COALESCE on Details comparisons in sp_Blitz.sql (~lines 10540-10541)

Test plan

• SQL Server 2022 CU25: both CheckID 179 modules appear in MARKDOWN after fix
• Agent Jobs Without Failure Emails grouping unchanged
• Hardware NUMA Config and Services multi-row groups unchanged

[BrentOzar]

Thanks for the pull request! Makes sense, looks good, merging into dev, will be in the next release with credit to you in the release notes.