Support $null for convertto-json (#10947)

ConvertTo-Json doesn't allow $null to be passed in even though it can be valid resulting json. Fix is to remove the check that the input is not null. NewtonSoft.Json handles it just fine.

test/powershell/Modules/Microsoft.PowerShell.Utility/ConvertFrom-Json.Tests.ps1

@@ -106,6 +106,17 @@ Describe 'ConvertFrom-Json Unit Tests' -tags "CI" {
         { $nestedJson | ConvertFrom-Json -AsHashtable:$AsHashtable } |
             Should -Throw -ErrorId "System.ArgumentException,Microsoft.PowerShell.Commands.ConvertFromJsonCommand"
     }
+
+    It 'Can convert null' {
+        'null' | ConvertFrom-Json | Should -Be $null
+        $out = '[1, null, 2]' | ConvertFrom-Json
+        $out.Length | Should -Be 3
+
+        # can't compare directly to array as Pester doesn't handle the $null
+        $out[0] | Should -Be 1
+        $out[1] | Should -Be $null
+        $out[2] | Should -Be 2
+    }
 }
 
 Describe 'ConvertFrom-Json -Depth Tests' -tags "Feature" {

test/powershell/Modules/Microsoft.PowerShell.Utility/ConvertTo-Json.Tests.ps1

@@ -62,4 +62,11 @@ Describe 'ConvertTo-Json' -tags "CI" {
 
         @{ 'abc' = "'def'" } | ConvertTo-Json @params | Should -BeExactly $expected
     }
+
+    It "Should handle null" {
+        [pscustomobject] @{ prop=$null } | ConvertTo-Json -Compress | Should -BeExactly '{"prop":null}'
+        $null | ConvertTo-Json -Compress | Should -Be 'null'
+        ConvertTo-Json -Compress $null | Should -Be 'null'
+        1, $null, 2 | ConvertTo-Json -Compress | Should -Be '[1,null,2]'
+    }
 }