Are there 2 kinds of $null in PowerShell?

Question

I've stumbled across an odd situation in PowerShell 5.1 (Windows 10) where ConvertTo-Json produces {} instead of null for an apparently null valued object:

[CmdletBinding()]
param()
function main() {
    $foo1 = $null
    Write-Verbose ("foo1 null? " + ($null -eq $foo1))
    $foo2 = try{1/0}catch{}
    Write-Verbose ("foo2 null? " + ($null -eq $foo2))
    $foo3 = try{1/0}catch{$null}
    return [PSCustomObject]@{"foo1"=$foo1; "foo2" = $foo2; "foo3" = $foo3} | ConvertTo-Json
}
Set-StrictMode -Version Latest
$ErrorActionPreference = "Stop"
main

Running this cmdlet -verbose shows two apparently $null variables which are serialized differently by ConvertTo-Json:

VERBOSE: foo1 null? True
VERBOSE: foo2 null? True
{
    "foo1":  null,
    "foo2":  {}
    "foo3":  null
}

What is going on here - are there different kinds of $null?

I can't to $foo2 | gm or $foo2.GetType() because $foo2 is null.