fix(ci): handle empty Pester shard lists in matrix build

Workflow-only changes produced @('') from Resolve-PesterCiShards,
which failed Mandatory [string[]] binding in Get-PesterCiShardMatrix.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: bolens

.github/workflows/test-pester.yml

@@ -57,7 +57,10 @@ jobs:
             }
           }
 
-          $include = @(Get-PesterCiShardMatrix -Shards $shards)
+          # Coerce to [string[]] so empty "no matching shards" stays an empty array
+          # (bare @() / $null can bind as '' and fail Mandatory [string[]]).
+          $shardList = [string[]]@($shards | Where-Object { -not [string]::IsNullOrWhiteSpace($_) })
+          $include = @(Get-PesterCiShardMatrix -Shards $shardList)
           $any = $include.Count -gt 0
           $matrix = if ($any) {
             @{ include = @($include) } | ConvertTo-Json -Compress -Depth 5

scripts/utils/code-quality/modules/PesterCiShardFilter.psm1

@@ -380,8 +380,9 @@ function Resolve-PesterCiShards {
         })
 
     if ($files.Count -eq 0) {
-        # Unary comma preserves empty [string[]] (bare `return @()` unwraps to $null).
-        return , [string[]]@()
+        # Emit nothing so call-site `@()` is a true empty array (Count 0).
+        # Unary-comma / -NoEnumerate empty [string[]] still becomes @('') under `@()`.
+        return
     }
 
     foreach ($ruleName in $rules.Keys) {
@@ -406,7 +407,7 @@ function Resolve-PesterCiShards {
 
     $ordered = [string[]]@($shards | Sort-Object)
     if ($ordered.Count -eq 0) {
-        return , [string[]]@()
+        return
     }
     return $ordered
 }
@@ -421,14 +422,22 @@ function Get-PesterCiShardMatrix {
     param(
         [Parameter(Mandatory)]
         [AllowEmptyCollection()]
+        [AllowEmptyString()]
+        [AllowNull()]
         [string[]]$Shards
     )
 
     $windowsShards = @(Get-PesterCiWindowsShards)
     $archShards = @(Get-PesterCiArchShards)
     $include = [System.Collections.Generic.List[object]]::new()
 
-    foreach ($shard in ($Shards | Sort-Object -Unique)) {
+    # Empty `@()` / `$null` / `''` can arrive from GHA when no shards match changed paths.
+    $shardNames = @(
+        $Shards |
+            Where-Object { -not [string]::IsNullOrWhiteSpace($_) } |
+            Sort-Object -Unique
+    )
+    foreach ($shard in $shardNames) {
         # Performance shards are Windows-only (historically flaky / slow on Ubuntu).
         if ($shard -notlike 'performance*') {
             $include.Add([pscustomobject]@{

tests/unit/test-runner/ci/test-runner-pester-ci-shard-filter.tests.ps1

@@ -77,6 +77,19 @@ Describe 'PesterCiShardFilter' {
         $matrix | Where-Object { $_.label -eq 'arch-latest' -and $_.shard -eq 'coverage-smoke' } | Should -Not -BeNullOrEmpty
     }
 
+    It 'Accepts empty or blank shard lists without throwing' {
+        @(Get-PesterCiShardMatrix -Shards @()) | Should -HaveCount 0
+        @(Get-PesterCiShardMatrix -Shards @('')) | Should -HaveCount 0
+        @(Get-PesterCiShardMatrix -Shards $null) | Should -HaveCount 0
+    }
+
+    It 'Returns a true empty array for workflow-only changes with no Pester shards' {
+        $shards = @(Resolve-PesterCiShards -ChangedFiles @('.github/workflows/codeql.yml'))
+        $shards.Count | Should -Be 0
+        @($shards | Where-Object { -not [string]::IsNullOrWhiteSpace($_) }) | Should -HaveCount 0
+        @(Get-PesterCiShardMatrix -Shards ([string[]]@($shards))) | Should -HaveCount 0
+    }
+
     It 'Exposes a curated Arch shard set' {
         $arch = Get-PesterCiArchShards
         $arch | Should -Contain 'unit-library'