Fix CSV and JSON export to handle null values correctly

[Copilot]

When exporting query results to CSV format, null values were being saved as the literal string "null" instead of blank cells. This behavior was inconsistent with Excel export functionality, where null values correctly appear as empty cells.

Problem

The issue occurred in the constructCsvString and constructJsonString methods in queryRunner.ts. Both methods were using the displayValue property directly without checking the isNull flag of DbCellValue objects. When a database column contained a null value, the displayValue would contain the string "null", which was then exported literally.

Solution

Modified both export methods to check the isNull property first:

CSV Export:

// Before: Always used displayValue
let displayValue = this.shouldRemoveNewLines()
    ? this.removeNewLines(x.displayValue)
    : x.displayValue;

// After: Check isNull first
let displayValue = x.isNull
    ? ""
    : this.shouldRemoveNewLines()
      ? this.removeNewLines(x.displayValue)
      : x.displayValue;

JSON Export:

// Before: Always used displayValue
let value = this.parseJsonValue(displayValue);

// After: Check isNull first
let value: any;
if (cellObject.isNull) {
    value = null;
} else {
    let displayValue = /* ... */;
    value = this.parseJsonValue(displayValue);
}

Testing

Added comprehensive test cases for both CSV and JSON exports that verify:

• Null values export as empty strings in CSV (e.g., "1,,3" instead of "1,null,3")
• Null values export as proper JSON null values (not "null" strings)
• No literal "null" strings appear in either export format

Impact

This change makes the CSV export behavior consistent with Excel export, where null database values appear as blank cells. The fix is minimal and surgical, only affecting the specific null value handling while preserving all existing functionality for non-null values.

Fixes #18528.

Warning

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[github-actions]

PR Changes

Category	Target Branch	PR Branch	Difference
Code Coverage	55.27%	56.49%	⚪ 0.00%
VSIX Size	4797 KB	4735 KB	🟢 -62 KB ( -1% )
Webview Bundle Size	3932 KB	3828 KB	🟢 -104 KB ( -2% )