Invoke-PkProcessBatch: accept $null -Commands as an empty batch

ParallelKit.Harness.ps1

@@ -197,6 +197,11 @@ Test-Assert ($seen.Count -eq 6) "-OnComplete fires for each command that ran"
 Test-Assert (-not $batch.Aborted) "a clean batch is not marked aborted"
 $wd = Invoke-PkProcessBatch -Commands @(@{ FilePath = $cmdExe; Arguments = '/c cd'; WorkingDirectory = $env:SystemRoot }) -MaxConcurrent 1
 Test-Assert ($wd.Results[0].Stdout.Trim() -ieq $env:SystemRoot) "a command's WorkingDirectory is honored"
+# A foreach over nothing yields $null, not an empty array - the usual way to build the list when no files match.
+$noCommands = foreach ($x in @()) { @{ FilePath = $cmdExe; Arguments = '/c exit 0' } }
+$emptyBatch = $null
+try { $emptyBatch = Invoke-PkProcessBatch -Commands $noCommands -MaxConcurrent 2 } catch { Write-Host "  empty batch threw: $_" }
+Test-Assert ($null -ne $emptyBatch -and $emptyBatch.Results.Count -eq 0 -and -not $emptyBatch.Aborted) "a batch given `$null (an empty foreach) returns no results"
 Write-Host ""
 
 # ==============================================================================

ParallelKit.ps1

@@ -1155,8 +1155,11 @@ function Invoke-PkProcessBatch {
     # Returns @{ Results; Aborted; ForceKilledCount }; each result is
     # @{ Command; ExitCode; Stdout; Stderr; StreamTimedOut; StreamError; StartError; Aborted },
     # every field always present - keep the three places that build one in sync.
+    # $Commands may be $null: a foreach that matches nothing yields $null, not
+    # an empty array, and an empty batch should just return no results.
     param(
         [Parameter(Mandatory)]
+        [AllowNull()]
         [AllowEmptyCollection()]
         [hashtable[]]$Commands,
         [int]$MaxConcurrent = 4,
@@ -1176,7 +1179,10 @@ function Invoke-PkProcessBatch {
     $jobBuilder = {
         param($group)
         $jobs = [System.Collections.Generic.List[hashtable]]::new()
-        foreach ($c in $Commands) { $jobs.Add(@{ FilePath = $c['FilePath']; Arguments = [string]$c['Arguments']; WorkingDirectory = [string]$c['WorkingDirectory']; Command = $c }) }
+        foreach ($c in $Commands) {
+            if ($null -eq $c) { continue }
+            $jobs.Add(@{ FilePath = $c['FilePath']; Arguments = [string]$c['Arguments']; WorkingDirectory = [string]$c['WorkingDirectory']; Command = $c })
+        }
         return $jobs
     }.GetNewClosure()