Fix File I/O NULL fallbacks for restricted permissions (#633)

[erikdarlingdata]

When sys.master_files is inaccessible, ISNULL now falls back to DB_NAME() and File_{id}.

Summary by CodeRabbit

• Bug Fixes
  • Enhanced file I/O statistics collection by implementing improved null-value handling and more robust fallback values when critical metadata such as database names, file names, and physical paths are unavailable or missing.
  • Refined data collection expressions to ensure complete and accurate file I/O statistics, preventing incomplete or malformed records in collected metrics.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between fd7f1e1 and 2d122ab.

📒 Files selected for processing (2)

• Lite/Services/RemoteCollectorService.FileIo.cs
• install/20_collect_file_io_stats.sql

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📝 Walkthrough

Walkthrough

Modified null-handling logic in file I/O statistics collection queries across two SQL files. Changed fallback values for database_name, file_name, file_type, and physical_name when metadata is missing or null.

Changes

Cohort / File(s)	Summary
File I/O Stats Collection Queries Lite/Services/RemoteCollectorService.FileIo.cs, install/20_collect_file_io_stats.sql	Adjusted null-handling and fallback values in metadata column selection: database_name now falls back to DB_NAME(vfs.database_id) instead of literal values; file_name now constructs fallback as 'File_' + CONVERT(nvarchar(10), vfs.file_id) instead of literals; file_type falls back to N'UNKNOWN'; physical_name wrapped with ISNULL() to ensure empty string when null.

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