【Powershell】変数がNullであるかどうかチェック

PS C:> $a = $null
PS C:> if($a -eq $null){write-host "null"}else{write-host "not null"}
null

PS C:> $a = "a"
PS C:> if($a -eq $null){write-host "null"}else{write-host "not null"}
not null