PowerShell commands that output nothing output the special [System.Management.Automation.Internal.AutomationNull]::Value singleton, which typically behaves like $null.

Unexpectedly, ConvertTo-Json doesn't treat it as $null, but treats it as an empty object, which is undesired.

Steps to reproduce

# Note: `. {}` yields the [System.Management.Automation.Internal.AutomationNull]::Value
# singleton, not $null.
[pscustomobject] @{ foo = . {}; bar = $null } | ConvertTo-Json

Expected behavior

{
  "foo": null,
  "bar": null
}

Actual behavior

{
  "foo": {},
  "bar": null
}

Environment data

PowerShell Core 6.2.0-rc.1
Windows PowerShell v5.1