PowerShell - How do I test for a variable value with “Set-StrictMode -version latest”

# all FAIL if $var is undefined under "Set-StrictMode -version latest"
if ( !$var ) { $var = "new-value"; }
if ( $var -eq $null ) { $var = "new-value"; }

if (!(test-path variable:var)) {$var = $null} # test for EXISTENCE & create
if ( !$var ) { $var = "new-value"; }           # test the VALUE

$var = & { Set-StrictMode -off; switch( $var ) { $null { "new-value" } default { $var } }}

& { Set-StrictMode -off; if (!$var) { set-variable -scope 1 var "new-value" }}

function set-Variable-IfMissingOrNull ($name, $value)
{
$isMissingOrNull = !(test-path ('variable:'+$name)) -or ((get-variable $name -value) -eq $null)
if ($isMissingOrNull) { set-variable -scope 1 $name $value }
}
set-alias ?? set-Variable-IfMissingOrNull
#...
## in use, must not have a leading $ or the shell attempts to read the possibly non-existant $var
set-VarIfMissingOrNull var "new-value"
?? varX 1

function test-variable
{# return $false if variable:$name is missing or $null
param( [string]$name )
$isMissingOrNull = (!(test-path ('variable:'+$name)) -or ((get-variable -name $name -value) -eq $null))
return !$isMissingOrNull
}
set-alias ?-var test-variable
if (!(?-var var)) {$var = "default-value"}

Set-Variable

-- ReadOnly: Cannot be deleted or changed without the Force parameter.
-- Constant: Cannot be deleted or changed. Constant is valid only when
creating a new variable. You cannot set the Constant option on an
existing variable.