Fix #2555: Should-BeNull treats empty pipeline input as null

src/functions/assert/General/Should-BeNull.ps1

@@ -32,6 +32,15 @@
 
     $collectedInput = Collect-Input -ParameterInput $Actual -PipelineInput $local:Input -IsPipelineInput $MyInvocation.ExpectingInput -UnrollInput
     $Actual = $collectedInput.Actual
+
+    # When a function returns no output and the result is piped to Should-BeNull,
+    # PowerShell sends an empty array @() through the pipeline. Treat empty pipeline
+    # input as $null since "no output" is effectively null.
+    # See https://github.com/pester/Pester/issues/2555
+    if ($collectedInput.IsPipelineInput -and $Actual -is [array] -and $Actual.Count -eq 0) {
+        $Actual = $null
+    }
+
     if ($null -ne $Actual) {
         $Message = Get-AssertionMessage -Expected $null -Actual $Actual -Because $Because -DefaultMessage "Expected `$null,<because> but got <actualType> '<actual>'."
         throw [Pester.Factory]::CreateShouldErrorRecord($Message, $MyInvocation.ScriptName, $MyInvocation.ScriptLineNumber, $MyInvocation.Line.TrimEnd([System.Environment]::NewLine), $true)

tst/functions/assert/General/Should-BeNull.Tests.ps1

@@ -9,8 +9,15 @@ Describe "Should-BeNull" {
         { 1 | Should-BeNull } | Verify-AssertionFailed
     }
 
-    It "Given empty array it fails" {
-        { @() | Should-BeNull } | Verify-AssertionFailed
+    It "Given empty array piped it passes (void function output is empty array)" {
+        # When a function returns no output, PowerShell sends @() through the pipeline.
+        # Should-BeNull treats this as $null since "no output" is effectively null.
+        # See https://github.com/pester/Pester/issues/2555
+        @() | Should-BeNull
+    }
+
+    It "Given empty array by parameter it fails" {
+        { Should-BeNull -Actual @() } | Verify-AssertionFailed
     }
 
     It "Returns the given value" {